I spent some time riding with a pro UHC rider last month, and he was a self described late bloomer... started racing at 23, turned pro at 24. Is 31 now. He went big fast though. Won Iron Horse as a cat 3, then turned around as a Cat 2 and won Gila the next spring (2007). I don't see any other road wins since (podiums, including nat'l crit champs), but still a pro... (to rkwaki's point about riding for the team).
He's extremely professional in conversation, went to college (science teacher), bought a home-town bike shop, plans to go back to teaching when done pro pedaling.