In brief: Cross-sectional studies of physical performance generally show a linear decline in maximum aerobic power (V O2max) with increasing age (about 1% to 2% per year). In the study described in this article, 15 men were serially followed for 20 years. They performed exercises consisting of walking, running, swimming, and cycling for an average of 3.6 days per week and requiring an energy expenditure of 2,104 kcal per week. Over the 20 years, directly measured V O2max declined 12% (from 44.4 to 38.9 ml·kg−1·min−1, or 0.27 ml·kg−1·min−1per year). Essentially no differences were seen in resting heart rate or arterial blood pressure. The authors conclude that physical training forestalls the decline in V O2max until at least age 65.