Actually, I suspect for some competitors who have aged, they would be better off in some ways fighting at that age, depending on their opponent. Whilst their bodies have aged, they have learnt more than they knew when they were at their physical peak. And since no one has the strength or endurance to rival the Mountain, they have to out wit him - an ability gained through life experience. Although it makes for great speculation, its almost cheating (if this had any actual rules) to give the competitors young strong bodies and give them credit for all the experience they gained in a lifetime.