Like everyone else said, he has always stayed in top shape, healthy diet, helps retain the most out of what you already have. Also as he's slowed down he has made himself more awkward for opponents, and he has adapted. Another thing has helped is that for a long time he didn't have a whole lot of good competition so he wasn't put through as many wars as other people we have seen. All that aside he is a great athlete who instead of relying on his ability decided to make the most out of it, I forgot another thing he had was a great chin which also helps.