Increasing age is associated with a decline in fat-free mass. The question is whether age-related changes in body composition can be delayed by an active life style. This analysis includes data where physical activity was assessed with doubly labeled water and body composition with hydrodensitometry or isotope dilution. Subjects were 136 women and 180 men over 20 years, who were tested in Maastricht University between 1983 and 1998. Increasing age was associated with lower activity levels and lower fat-free mass. After controlling for age there was no longer any association between physical activity and fat-free mass. A few exercise intervention studies showed that elderly subjects compensate for exercise training by a decline in spontaneous physical activity, in contrast to younger subjects. Although no effect of habitual activity level on changes in body composition are observed, training has a positive effect on muscle function. Elderly subjects with relatively high levels of physical activity are not different from those with low activity levels, as far as fat-free mass and fat mass are concerned. However, training might delay the age-induced impairment of personal mobility associated with a reduction in physical activity.