Charles Xavier

Charles Xavier aka Professor X.

20 or under
He says his parents worked on the first A-Bomb project, implying that he was born afterward and that this caused his mutation.

https://en.wikipedia.org/wiki/Manhattan_Project began in 1942, so a child conceived then could be born in 1943 meaning by 1963 the oldest Professor X could possibly be is 20 years old when he brings Jean Gray to join the X-Men.