I'm not convinced that he was a late bloomer. He had a sterling lengthy amateur career culmulating in the greatest prize of the amateur game....a gold medal which he won when he was only 23.
He probably hit his peak around 1997 when he was 32; which is about right for a larger heavyweight. But there is no doubt that he was one of the top five heavyweights in the world from the age of about 26 onwards.
Athletically he was probably prime around 27 when he met Ruddock. But He improved technically as a fighter after 1996 due to him working with Manny Steward.