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Aging of Population
Leonid A. Gavrilov and Patrick Heuveline



This is a manuscript of our article in The Encyclopedia of Population. New York, Macmillan Reference USA, 2003.


[Note: This original manuscript is slightly different from the final publication because of small editorial changes.]




Reference to the published article:


Gavrilov L.A., Heuveline P.

“Aging of Population.”

In: Paul Demeny and Geoffrey McNicoll (Eds.)

The Encyclopedia of Population. New York, Macmillan Reference USA, 2003

Available at:


Aging of population (also known as demographic aging, and population aging) is a summary term for shifts in the age distribution (i.e., age structure) of a population toward older ages.  A direct consequence of the ongoing global fertility transition (decline) and of mortality decline at older ages, population aging is expected to be among the most prominent global demographic trends of the 21st century. Population aging is progressing rapidly in many industrialized countries, but those developing countries whose fertility declines began relatively early also are experiencing rapid increases in their proportion of elderly people. This pattern is expected to continue over the next few decades, eventually affecting the entire world.  Population aging has many important socio-economic and health consequences, including the increase in the old-age dependency ratio.  It presents challenges for public health (concerns over possible bankruptcy of Medicare and related programs) as well as for economic development (shrinking and aging of labor force, possible bankruptcy of social security systems).


Defining and measuring population aging

As the study of population aging is often driven by a concern over its burdening of retirement systems, the aging of population is often measured by increases in the percentage of elderly people of retirement ages. The definition of retirement ages may vary but a typical cutoff is 65 years, and nowadays a society is considered relatively old when the fraction of the population aged 65 and over exceeds 8-10%. By this standard, the percentage of elderly people in the United States stood at 12.6% in 2000, compared with only 4.1% in 1900 and a projected increase to 20% by the year 2030.

A related measure of population aging is the elderly dependency ratio (EDR): the number of individuals of retirement ages compared to the number of those of working ages. For convenience, working ages may be assumed to start at age 15, although increasing proportions of individuals pursue their education beyond that age and remain, meanwhile, financially dependent, either on the state or, increasingly, on their parents or bank managers. The ratio of the elderly dependent population to the economically active (working) population is also known as old-age dependency ratio, age-dependency ratio or elderly dependency burden and is used to assess intergenerational transfers, taxation policies, and saving behavior.

Another indicator of the age structure is the aging index (sometimes referred to as the elder-child ratio), defined as the number of people aged 65 and over per 100 youths under age 15. In 2000, only a few countries (Germany, Greece, Italy, Bulgaria, and Japan) had more elderly than youth (aging index above 100). By 2030, however, the aging index is projected to exceed 100 in all developed countries, and the index of several European countries and Japan are even expected to exceed 200. To date, aging indexes are much lower in developing countries than in the developed world, but the proportional rise in the aging index in developing countries is expected to be greater than in developed countries.

These indicators of population aging are mere head-count ratios (HCR), that is, they simply relate the number of individuals in large age categories. These indicators fail to take into account the age distribution within these large categories, in particular among the elderly. When the fertility and mortality trends responsible for population aging have been fairly regular over time, the population growth is positively correlated with age (i.e., the oldest age groups are growing fastest). This implies that if the proportion of the population over age 65 is increasing, within that 65-and-over population the proportion over, say, age 80 is also increasing. As health, financial situation, and consumption patterns may vary greatly between 65 year-olds and 80 year-olds, simple ratios conceal important heterogeneity in the elderly population. Increasingly, attention is paid to the "oldest olds" (typically age 80 and over). A long-time subject of curiosity, the number of centenarians is growing even faster. Estimated at 180,000 worldwide in 2000, it could reach 1 million by 2030 (United Nations 2001).

The second class of indicators for population aging is the group of statistical measures of location (median, mean and modal ages of population). The median age -- the age at which exactly half the population is older and another half is younger -- is perhaps the most widely used indicator. For the year 2000, the median age in the United States was 36 years, a typical age for most developed countries and twice the median age for Africa (United Nations 2001). Because it is more sensitive to changes at the right-hand tail of the age distribution (i.e., the oldest old ages), the mean age of population might in fact be preferred to the median age to study the dynamics of population aging.

Since population aging refers to changes in the entire age distribution, any single indicator might appear insufficient to measure it. The age distribution of population is often very irregular, reflecting the scars of the past events (wars, depression etc.), and it cannot be described just by one number without significant loss of information. Were the age distribution to change in a very irregular fashion over the age range, for instance, much information would be lost by a single-index summary. Therefore, perhaps the most adequate approach to study population aging is to explore the age distribution through a set of percentiles, or graphically by analyzing the population pyramids. Demographers commonly use population pyramids to describe both age and sex distributions of populations. Youthful populations are represented by pyramids with a broad base of young children and a narrow apex of older people, while older populations are characterized by more uniform numbers of people in the age categories.

Figures 1-5 About Here


Demographic determinants of population aging

To understand the demographic factors that cause population aging, demographers often refer to stable populations (Preston et al. 2001). This population model assumes that age-specific fertility and mortality rates remain constant over time, and this results in a population with an age distribution that stabilizes and eventually becomes time invariant as well. Conversely, this theoretical model suggests that any change in age structure, and population aging in particular, can only be caused by changes in fertility and mortality rates. The influence of changes in fertility rates on population aging is perhaps less intuitive than that of mortality rates. Everything else constant, however, a fertility decline reduces the size of the most recent birth cohorts relative to the previous birth cohorts, hence reducing the size of the youngest age groups relative to that of the older ones.

 The effects of changes in mortality rates on population aging appear more intuitive, but are in fact more ambiguous. If increases in the human life span are correctly linked to population aging, reductions in mortality rates do not necessarily contribute to population aging. More specifically, mortality declines among infants, children and persons younger than the population mean age tend to lower the population mean age.  A moment of thought suggests that indeed a reduction of neonatal mortality (i.e., death in the first month of life) adds individual at age 0 and should lead to the same partial alleviation of population aging as an increase in childbearing.

Population aging is thus related to the demographic transition, that is the processes that lead a society from a demographic regime characterized by high rates of fertility and mortality to another one with lower fertility and mortality rates. In the course of this transition, the age structure is subjected to different influences. In the typical sequence, the transition begins with successes in preventing infectious and parasitic diseases that benefit infants and young children most. The resulting improvement in life expectancy at birth occurs while fertility tends to remain unchanged, thereby producing large birth cohorts and an expanding proportion of children relative to adults. Other things being equal, this initial decline in mortality generates a younger population age structure.

After initial and sometimes very rapid gains in infant and child mortality have been achieved, further mortality declines increasingly benefit older ages and are eventually accompanied by fertility declines. Both changes contribute to reverse the early effect of mortality decline on the age structure, and this synergy is known as the double aging process. This corresponds to the experience of most developed countries today, but further decomposition suggest that their history of declining mortality is the dominant factor in current aging (Preston, Himes and Eggers 1989). Mortality declines continue in these countries and the decrease in mortality rates among the oldest-old (85+ years) has actually accelerated since the 1950s (Gavrilov, Gavrilova, 1991). This latest phase of mortality decline, which is concentrated in the older age groups, is becoming an important determinant of population aging, particularly among women.

The rate of population aging may also be modulated by migration. Immigration usually slows down population aging (in Canada and Europe, for example), because immigrants tend to be younger and have more children.  On the other hand, emigration of working-age adults accelerates population aging, as it is observed now in some Caribbean nations. Population aging in these countries is also accelerated by immigration of elderly retirees from other countries, and return migration of former emigrants who are above the average population age.  Some demographers expect that migration will have a more prominent role in population aging in the future, particularly in low-fertility countries with stable or declining population size.  The effects of migration on population aging are usually stronger in smaller populations, because of higher relative weight (proportion) of migrants in such populations.


Dynamics of population aging

The current level and pace of population aging vary widely by geographic region, and usually within regions as well, but virtually all nations are now experiencing growth in their numbers of elderly residents (for selected regions and countries, see Table 1).  The percentage of world population aged 65 and over only increased from 5.2% in 1950 to 6.9% in 2000. In Europe, however, the proportion is 14.7% in 2000. For a long time, the highest proportions where found in Northern Europe (e.g., 10.3% in Sweden in 1950), but had moved South by 2000 (18.1% in Italy). The proportions of elderly are lower outside of Europe with the notable exception of Japan where it increased from 4.9% in 1950 to 17.2% in 2000.  The age structure of the United States continues to be marked by the large birth cohorts of the baby boom (people born from 1946 through 1964), not yet aged 65.  The proportion of the elderly population in the U.S., 12.3% in 2000, hence remains low compared to the developed-country standards.  .


Table 1 About Here


Population aging has the following notable features:

(1) The most rapid growth occurs in the oldest age groups – the oldest-old (80+ or 85+ years) and centenarians (100+ years) in particular.  In other words, population aging is becoming “deeper” with preferential accumulation of particularly old and frail people.

(2) Population aging is particularly rapid among women, resulting in “feminization” of population aging (because of lower mortality rates among women).  For example, in the United States, there were 20.6 million older women and 14.4 million older men in 2000, or a sex ratio of 143 women for every 100 men.  The female to male ratio increases with age reaching 245 for persons 85 and over. 

(3) Another consequence of lower female mortality is the fact that almost half of older women (45%) in 2000 were widows, thus living without spousal support. 

(4) Population aging also causes changes in living arrangements resulting in increasing number of older people living alone (about 30% of all non-institutionalized older persons in 2000 lived alone in the United States). 

(5) Since older persons have usually lower income and a higher proportion of them are living below the poverty line, population aging is associated with poverty, particularly in developing countries.


Projections of population aging in the 21st century

Future population aging will depend on future demographic trends, but most demographers agree that the fertility and mortality changes that would be required to reverse population aging in the coming decades are very unlikely. According to current population forecasts, population aging in the first half of this century should exceed that of the second half of the 20th century. For the world as a whole, the elderly will grow from 6.9% of the population in 2000 to a projected 19.3% in 2050 (Table 1). In other words, the world average should then be higher than the current world record. All regions are expected to see an increase, although it should be milder in some regions, such as Africa where the projected increase is from 3.3% in 2000 to 6.9% in 2050. But in Latin America and the Caribbean, the increase should be from 5.4% in 2000 to 16.9% in 2050, higher than the current European average. The increase should be even more spectacular in China: from 6.9% in 2000 to 22.7% in 2050.

If population aging is thus far from limited to the most developed regions, the countries of these regions will likely continue to experience the highest proportions ever known. The forecasts suggest 29.2% of elderly in the European population as a whole, but more than 30% in a number of specific European countries, and perhaps as much as 36.4% in Japan. Again, the forecasted increase from 12.3% in 2000 to 21.1% in 2050 appears less dramatic in the U.S. than in other most developed countries.

There is of course some uncertainty with any forecast, but it is important to note that previous population forecasts underestimated rather than overstated the current pace of population aging.  Before the 1980s the process of population aging was considered as an exclusive consequence of fertility decline and it was predicted that the pace of population aging would decrease after stabilization of fertility rates at some low levels.  Rapid decline in old-age mortality observed in developed countries in the last decades of the 20th century significantly accelerated population aging.  Now the old-age mortality trends are becoming the key demographic component in projecting the size and composition of the world's future elderly population.  Current and future uncertainties about changing mortality may produce widely divergent projections of the size of tomorrow's elderly population.  For example, the U.S. Census Bureau's middle-mortality series projection suggests that there will be 14.3 million people aged 85 and over in the year 2040, while the low-mortality (i.e., high life expectancy) series implies 16.8 million.  Alternative projections, using assumptions of lower death rates and higher life expectancies, have produced estimates from 23.5 to 54 million people aged 85 and over in 2040 in the United States (see Kinsella, Velkoff, 2001). 


Social and economic implications of population aging

While population aging represents, in one sense, a success story for mankind (massive survival to old ages has become possible), it also poses profound challenges to public institutions that must adapt to a changing age structure. 

The first challenge is associated with dramatic increase in the older retired population relative to the shrinking population of working ages, which creates social and political pressures on social support systems.  In most developed countries, rapid population aging places a strong pressure on social security programs.  For example, the U.S. social security system may face a profound crisis if no radical modifications are enacted.  Cuts in benefits, tax increases, massive borrowing, lower cost-of-living adjustments, later retirement ages, or a combination of these elements are now discussed as the possible painful policies, which may become necessary in order to sustain the pay-as-you-go public retirement programs such as Medicare and Social Security.

Population aging is also a great challenge for the health care systems.  As nations age, the prevalence of disability, frailty, and chronic diseases (Alzheimer’s disease, cancer, cardiovascular and cerebrovascular diseases, etc.) is expected to increase dramatically.  Some experts raise concerns that the mankind may become a “global nursing home” (Eberstadt, 1997).

The aging of the population is indeed a global phenomenon that requires international coordination of national and local actions.  The United Nations and other international organizations developed recommendations intended to mitigate the adverse consequences of population aging.  These recommendations include reorganization of social security systems, changes in labor, immigration and family policies, promotion active and healthy life styles, and more cooperation between the governments in resolving socioeconomic and political problems posed by population aging.

On the positive side, the health status of older people of a given age is improving over time now, because more recent generations have a lower disease load.  Older people can live vigorous and active lives until a much later age than in the past and if they're encouraged to be productive, they can be economic contributors as well.  Also the possibility should not be excluded that current intensive biomedical anti-aging studies may help to extend the healthy and productive period of human life in the future (de Grey et al., 2002).



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Eberstadt, N. 1997.  “World population implosion?”  Public Interest, 129: 3-22.

Gavrilov, Leonid A., and Natalia S. Gavrilova. 1991.  The Biology of Life Span: A Quantitative Approach.  NY, etc.: Harwood Academic Publ..

Kinsella, Kevin, and Victoria A. Velkoff. 2001. An Aging World: 2001. U. S. Census Bureau, Series P95/01-1, Washington, DC: U.S. Government Printing Office.

Lutz, Wolfgang, Warren Sanderson, and Sergei Scherbow. 2001. “The end of world population growth.” Nature  412: 543-545.

Preston, Samuel H., Christine Himes, and Mitchell Eggers.  1989.  “Demographic conditions responsible for population aging.”  Demography  26: 691-704.

Preston Samuel H., Patrick Heuveline, and Michel Guillot. 2001. Demography. Measuring and Modeling Population Processes. Oxford: Blackwell.

United Nations 2001. World population prospects: the 2000 revision. New York: United Nations.

Table 1.  Dynamics of Population Aging in the Modern World

Observed and Forecasted Percentages of the Elderly (65+ years) in Selected Areas, Regions, and Countries of the World: 1950, 2000 and 2050.

Major Area, region and country













Latin America and the Caribbean





































Source: United Nations 2001.



Figure 1. Youthful population.



Figure 2. Aged population.



Figure 3. Intermediate population.




Figure 4. Projected extremely old population.




Figure 5. Projected old population.