I discussed this with a friend who has a masters in Exercise Physiology. Basically, your body functions like a machine, and just like a machine you only have so many cycles on the parts before they break down. Your body however, vs a machine, can heal itself. The older you get, the longer it takes for the body to repair itself and eventually it can't make repairs as fast you break it down.
Your own physical peak can come at different times based on several things. One, naturally when you completely develop. A great example of this is Mike Tyson and Muhammed Ali. Tyson physically was completely filled out by age 20, Muhammed Ali was much later than that.
That being said, a second factor is how much damage you put your body through. Generally speaking, the life of a quarterback that runs the ball and gets hit a lot is much less than a Dan Marino type who stays in the pocket and lets the line take all the hits. Most people in combat sports who build a career from taking tons of damage don't last long because subjecting your body to blow after blow with virtually no rest or time for the body to catch up means eventual injuries that your body won't be able to completely heal from.
I know with rock climbing and MMA, I want to keep doing them for a long time. That's why when I see a bunch of 18 year olds at the climbing gym going for hours a day, six days a week, jumping for holds and catching them with one hand I always tell them to slow down. I've seen plenty of climbers who pop tendons, break ankles, and rip rotator cuffs in their early 20's from overuse and they never get full range of motion back. And don't get me started on the horrible conditioning practices that go on in Mixed Martial Arts gyms.