I have noticed a lot of athletes hit their prime when they are in their 20s and that by the time they reach their 30s they are past it and start to decline. Great athletes tend to prolong their careers by adjusting their game strategy, but even so they can't do the things they used to do when they were younger. Examples of this are abundant in the NBA, NFL, MLB, NHL, football, etc.
Why do athletes suddenly have such a drop off in performance? What happens to their bodies, muscles, etc that prevents them from doing what they once did? Can someone explain the biological/physiological reason for this?