It’s no secret that the body wears down as it ages, and for competitive athletic events, you have a limited shelf life before you physically cannot compete at the same level as those who are in a greater physical condition.
There are quarterbacks right now who are increasingly beginning to play deeper into their late 30s and early 40s than ever before. Much of that has to do with the way the game and the rules have evolved that makes it easier for them to do so. But your skills and physical abilities still diminish over times.
Peyton Manning wasn’t the same player at the end of his career.