Overtraining isn't prevented

If you want to become an Olympic champion in female gymnastics, you have to start training 6 or younger; and you have to train long hours. And by 10 you would have to be able to do quite complicated routines and moves.

It doesn't matter what the age bans are - you cannot start training later because key abilities are learned in preschooler age that cannot be learned later to the same level, because of both physical and neurological development.

You can ban under-14, or under-16, or under-25 if that is your whim and it doesn't matter: the gymnasts that make it to the Olympics and that win the medals will always be the ones that trained since 6 or younger and for very long hours.

Besides, other sports are more stressful for the body (especially at an Olympic level) and they have never been seen under such a light: wrestling, weightlifting, of course, boxing, and arguably long distance running.

In the end, the argument about bodily stress should be immediately contradicted by the Olympic maxim "faster, higher, stronger": to be faster, higher and stronger, in any sport, is stressful and to a point even "anti-natural". But if you wanted to make Games with people who had no particular training stress then they wouldn't be the Games of the truly faster, higher, stronger.

Only Muhammad Ali can answer to himself whether his brilliant career was worth his Parkinson's. But nobody who have watched his fights can put in doubt his status as a sporting giant.