Physiological functional capacity (PFC) is defined here as the ability to perform the physical tasks of daily life and the ease with which these tasks can be performed. For the past decade, we have sought to determine the effect of primary (healthy) adult human aging on PFC and the potential modulatory influences of gender and habitual aerobic exercise status on this process by studying young adult and Masters athletes. An initial approach to determining the effects of aging on PFC involved investigating changes in peak exercise performance with age in highly trained and competitive athletes. PFC, as assessed by running and swimming performance, decreased only modestly until age 60-70 yr but declined exponentially thereafter. A progressive reduction in maximal O2 consumption (V(O2 max)) appears to be the primary physiological mechanism associated with declines in endurance running performance with advancing age, along with a reduction in the exercise velocity at lactate threshold. Because V(O2 max) is important in mediating age-related reductions in exercise performance and PFC, we then investigated the modulatory influence of habitual aerobic exercise status on the rate of decline in V(O2 max) with age. Surprisingly, as a group, endurance-trained adults appear to undergo greater absolute rates of decline in V(O2 max) with advancing age compared with healthy sedentary adults. This appears to be mediated by a baseline effect (higher V(O2 max) as young adults) and/or a marked age-related decline in exercise training volume and intensity (stimulus) in endurance-trained adults. Thus the ability to maintain habitual physical activity levels with advancing age appears to be a critical determinant of changes in PFC in part via modulation of maximal aerobic capacity.