While both the immune and the endocrine systems are undoubtedly involved in aging, researchers continue to search for the mechanisms to explain their effects. One approach to studying aging, caloric restriction, is expected to yield some clues. The fundamental reasons are not yet understood. It may have something to do with DNA repair rates, or free radical levels, or hormonal balance, or cell senescence, or all of these plus other mechanisms. What is known is that the mice live on restricted diets.
Individuals age at extremely different rates.
In fact even within one person, organs and organ systems show different rates of decline. However, some generalities can be made, based on data from the Baltimore Longitudinal Study of Aging. It is important to remember that these statements do not apply to all people. It grows slightly larger with age. Maximal oxygen consumption during exercise declines in men by about 10 percent with each decade of adult life and in women, by about 7.5 percent.