Around the mid-30s, on average. A Karolinska Institutet study that followed 427 Swedes born in 1958 from age 16 to 63 found that aerobic fitness and muscular endurance peaked between ages 26 and 36, and performance began declining around age 35; explosive muscle power peaked even earlier. The decline starts slowly and speeds up with age, and by 63 average performance had fallen roughly 30% to 48% from peak, with similar rates in men and women.
Exercise did not stop the decline, but people who became physically active as adults improved their physical capacity by about 5% to 10%. The findings were published in the Journal of Cachexia, Sarcopenia and Muscle.
Image: Guy jogging by sign. Photo by Guillaume de Germain (Unsplash), CC0, via Wikimedia Commons.