Once a fighter has passed their prime?

Is there anyway they can regain it? Tyson's prime was from 20 - 24 Why when he come out of his prime could he not discover his past form? It wasn't asif he was too old to rediscover it. Was it physical or mental? or both because of his prison stint. If Tyson come out of prison and got back with Kevin Rooney and the set up he had back then would we have seen the Tyson of 85-90 Is there any fighters you know of that have been brilliant gone through a bad spell got out of the bad spell and gone on to be great again? Or isit once you've reached the top of the mountain the only way is down? you would have to be a pretty special fighter to turn around and climb back upto the top once again.