Take note that they are now in a fantasy world with magic powers, so our understanding of human limits need to change. Their bodies probably grow endlessly (with of course individual differences, he is lauded as a genius/monster by Rose in this regard) as long as you are able to train and heal. Time to heal and recover is basically omitted, and he can train endlessly without risk of injury, even doing far more extreme methods that no one other than those that can heal themselves as well as him and bosslady can do.