My brother started playing piano when he was 6-7 years old. Now he is 17 and he still plays it and he is fucking good (annoying to hear it everyday though but man, he is good). And he has never had any lesson, he can listen to an instrumental by ear and spend some little time and then play it on the piano. He has also made some songs himself (without vocals).
He became really good at it already on the age of 10 or something, so it wasn't that many years.
He picked up a guitar a couple years ago I think, and has done pretty nice on it aswell. That without lessons aswell.











