Some of us seem old at 50. Others are still frisky at 90. The heterogeneity of old folks is taken by some to be one of the identifying characteristics of the aging process. I gave a major lecture, "The Plasticity of Aging," on this topic last July at Trinity College in Dublin.
Most old people are dead. Others aren't. Some are fit. Some are frail. Some are a liability, others are an asset. Just how fast do we age?
This is certainly a far-reaching, compelling, and important question. Its answer remained obscure until a friend of Dad's, Nathan Shock, in Baltimore published his observations in 1968 that surveyed a large group of older subjects and checked out nine bodily functions: heart, kidney, lung, muscle, etc. The range of their decrements ranged widely, suggesting a varying rate of age change. Later analyses, however, revealed a large artifact. Nathan's data were compromised by the inclusion in his cohort of many persons with pathologies obviating his conclusion of a basic rate of aging.
Recognizing this defect my doctor son, Walter, and I published a paper in the Journal of Gerontology in 1996 entitled "How fast do we age; exercise performance over time as a biomarker." The Masters athlete emerges as the ideal model for a disease free subject.
Accordingly, we obtained the world age performance records for running, swimming, and rowing. We argued that while the records represent specific performance it is not the absolute levels that are of interest. Rather, actually it is the rate of change. When we plotted the decay slopes over time there was a striking coincidence, indicating that 0.5 percent per year was the decay rate for all events. This linear worsening held until age 75. After that the performances deteriorate more rapidly. But from ages 25 to 75, 0.5 percent was the presumed rate of aging. No single organ function could deteriorate faster than this, or it would become rate limiting.
It was striking as well that this rate of 0.5 percent for athletic performance was similar to that of Kasch, et al, who recorded a 0.5 percent per year fall in oxygen transport capacity in fit persons.  However, the major point is that unfit people showed a decline rate of 2 percent per year. So fitness is equivalent to 1.5 percent per year difference. A fit person of 70 is biologically the same as an unfit person of 40.
Other data showed that over a broad range of body functions in healthy subjects, from the cellular level to the whole body, all also decrease at the rate of 0.5 percent per year. Probably the most notable of these is maximum pulse rate, which is calculated as 220 minus your age which is, in fact, 0.5 percent per year.
So doctor son Walter and I proclaim that the true rate of age change in fit persons is a half a percent point per year. This surely is stately and scarcely notable over a short period of time. But lack of fitness and the incursion of various diseases accelerates the deterioration so that functional distress is noted too soon.
Age has been held accountable for too much. Remember the punch line of the old joke," But Doc, the other leg is just as old and it is fine."
Use it or lose it, faster.
1. Kasch PW, Boyer JL, et al. "Effect of physical activity and inactivity on aerobic power in older men ( a longitudinal study)." J. Physician Sports Med 1990 18: 73-83