Clinton Woods-22 Luis Firpo-23 Rocky Marciano-23 Ken Norton-24 Dwight Muhammad Qawi-25 Jess Willard-27 Ron Lyle-30
I’m no hardcore boxing fan, but the most recent fighter I know of doing this was Sek-C Sergio. I think he was 20.
Tough to say, maybe long distance running or something. I don't believe it would be possible to pick up a team sport as an adult and turn pro, you need to gradually jump levels.