For me, it was at 38 years of age. Around that time, I found that my ability to handle high running mileage began to diminish and that my finish times in races (5k to marathon) began to slow. Although I cycle a bit, my first love is running. After 35 years running, racing and coaching high school and college runners, I disagree with the assertion stated above that a person is at her/his physical peak at 18 years. Perhaps it is a question of semantics and what one means by "fitness" or "strength". For serious distance runners, it takes years and years of gradually increasing work loads for one to achieve maximum aerobic capacity/fitness. The apex of that conditioning process occurs for most in their middle to late 20s while others only arrive at that point in their early 30s.