Rocky Marciano and Hasim Rahman are two guys that come to mind right off the bat that didn't take up boxing until their 20's. I'm sure there are better examples, but I didn't put a ton of research into it.
He hasn't won any world titles but Matt Skelton didn't start boxing until he was about 40 or somin'. Considering he is heavyweight commenwealth champ he hasn't done to bad.
He had a good kickboxing career before hand though, it's not like marciano who was pursuing a baseball career but then decided to be a fighter.
