They (top athletes) are also often physically devastated at age 40 and suffer from chronic issues for the rest of their lives. The incentives for top athletes are not conducive to a healthy life at all.
I mean getting exercise is definitely good for you and in a choice between too much and too little I think you're better off overdoing it, but just constantly burning your metabolism at full steam seems to age you faster, couple that with the impact caloric deficit is supposed to have on aging and at least in my mind there's a pretty clear connection between the two.
First of all, a healthy body is very good at adaptation, i.e. if properly trained, your bones, muscles and other tissues get thicker and less prone to tearing, breakage etc.
Second, you will be surprised at how much pressure could daily activities put on your tissues, e.g. your vertabrae disks get higher pressure when you are wobbly sitting than standing even with some additional load. And in my experience people who train their body for extreme conditions have less problems in their elderly than people who never trained much at all.
And as you've noticed, most sportspeople have troubles with their serious traumas, not due to routine trainings. Plus of course this traumatization level varies from sport to sport.
In this friends case it was the NFL part that caused the damage. Not the fitness part.
I once did a boxing class. The day we did drills with with those boxing pads that require 2 hands to hold up, my head was throbbing for the rest of the day. Tried it again and same thing. Then I quit. Clearly I cannot take a serious punch.
I know several pro swimmers though and haven't heard of any serious issues there, so i guess it's from the impacts and constant running.
Me, like the good nerd, I am developed back problems instead