What is aging, how does it occur, and how does it express itself in human clinical disease?

Aging is a summary term for a set of processes, which contribute to health deterioration and ultimately to death with the passage of time (calendar age).

In other words any process, which contributes to age-related decline in performance, productivity and health is a component of the aging process that deserves our attention and intervention.

You can think of aging as a group of processes responsible for such manifestations as increasing risk of  frailty, disability, morbidity (for age-related degenerative diseases in particular)  and ultimately increasing mortality rates.

This interpretation of aging is consistent with general definition of aging system in mathematical reliability theory and reliability engineering as a system demonstrating age-dependent increase in failure rates.  Failure is such an outcome when the system deviates from optimistically anticipated and desired behavior ("fails").

The main problem with studying aging is that it is the many-headed monster, with many types of failures and often with multiple failures. Therefore an attempt to describe this complex multidimensional phenomenon through the change of just one index  -- dubbed as "biological age", or "physiological age", or "real age" may be a misleading and even a deceptive oversimplification. More adequate scientific language to describe the aging phenomenon could be found in general system theory and in reliability theory in particular.

Interestingly, the reliability theory predicts that a system may deteriorate with age even if it is built from non-aging elements with constant failure rate. The key issue here is the system's redundancy for irreplaceable elements, which is responsible for the aging phenomenon. In other words, each particular step of system destruction/deterioration may seem to be apparently random (no aging, just occasional failure by chance), but if a system failure requires a sequence of several such steps (not just a single step of destruction), then system as a whole may have an aging behavior.

Why is this important?  Because the significance of beneficial anti-aging interventions is often undermined by claims that these interventions are not proven to delay the process of aging itself, but instead that they simply delay or "cover-up" some particular manifestations of aging.

In contrast to these pessimistic views, the reliability theory says that there may be no specific underlying elementary "aging process itself" -- instead aging may be largely a property of  redundant system as a whole, because it has a network of destruction pathways each being associated with particular manifestations of aging (types of failure). Therefore, we should not be discouraged by only partial success of each particular anti-aging intervention, but instead we can appreciate an idea that we do have so many opportunities to oppose aging in numerous different ways.

Thus, the efforts to understand the routes and the early stages of age-related degenerative diseases should not be discarded as irrelevant to understanding of the "true biological aging".   On the contrary, the attempts to build a Berlin wall between biogerontology and clinical medicine are counterproductive. After all, the main reason why people are really concerned about aging is because it is related to health deterioration and increased morbidity. The most important pathways of age changes are those that make older people sick.

Cited from:  "Pieces of the puzzle. An Interview with Leonid A. Gavrilov, Ph.D."
Journal of Anti-Aging Medicine, 2002, 5(3): 255-263. 
Interview (pdf file)

Further reading: