Warning: old, old thread
Nakayama Noriyuki is famous for taking up Go late, and also becoming a professional very late as well (30).
In the AGA E–Journal of 2004, it was reported:
Nakayama sensei began playing go when he was 15 years old, just after the end of the Second World War. He had watched the game often, and knew the rules, but had never held the stones himself. “Go was not a children’s game. Of all the Japanese pros, I probably have the latest start.”
Sensei’s Library, though, claims that he began at 13. This tallies better with his birth date, which is given as September 3, 1932, supposing as SL says that he took it up in 1945. I wouldn’t be surprised if there had been some confusion over Oriental age notation, which can produce “ages” a year or even two older than in the Western system.
I think it’s safe to say that one must be exceptionally gifted to, in the 21st century, reach the professional level without having played seriously before one’s tenth birthday. Some of the strongest professionals, like Cho Hunhyun, were even able to qualify before that time.
Outside of Go, Joseph Blackburne was able to become a chess professional, and even a top one, after learning the game at 17 or 18 “in, say, 1859.” I thought I’d read that Paul Keres (b. 1916) was forbidden to study chess for a time as a child, but Wikipedia doesn’t mention this, and in any case by 1930 (13 or 14) he was the Estonian schoolboy champion.
I’ve also read about some classical (ie. pre-1900s) Go players who were noticed as particularly strong children but, for financial reasons, weren’t able to train as insei until teenagers or young adults.