Football is a young man’s game and if you look at the NFL compared to the other three major sports in the United States then you will see superstars retiring at a much younger age. Stars like Calvin Johnson and Andrew Luck retired during a period in which other athlete’s primes would start.
It is more than just the Luck/Johnson examples, though. We see starts in the NFL decline at a younger age than most other athletes. While it is not true for all NFL players (especially quarterbacks), some of the best in the league will be out of the league by their mid-30s.