Nice point. I think they still started very young. I wonder if there are examples of late bloomer mathematicians, that did great work despite not learning any advanced maths before their 30s or 40s.
Raymond Smullyan got his BSc in his late 30s and went on to write highly acclaimed textbooks and teach at universities, although I'm not exactly sure about his research accomplishments: https://en.wikipedia.org/wiki/Raymond_Smullyan
EDIT: see also this list https://mathoverflow.net/questions/3591/mathematicians-who-w...