Liebling in 'Sweet Science' wrote very well about aging Louis, in the first chapter if I recall correctly.
I think he aged brilliantly. He had a long and successful reign as champ, and during his comeback he was probably still in the top 4 or 5 ighters in the division. He thrashed Lou Nova (a guy Marciano looked really clumsy against) and although obviously he was not the same guy he had been years before, he was still a dangerous opponent. In the build-up to the Marciano fight his trainer (for the life of me I can't recall his name right now) was very pleased with how Joe was coming along in training. He felt he was in better shape than at any other point during his comeback. Hearing such reports emanating from the Louis camp was a cause for concern for the Marciano camp. They didn't like what they were hearing. Whether Joe really was in such good shape or it was propoganda I can't say, but the Marciano camp took Joe very, very seriously indeed.