Richard Guy said that he "began to think of [himself] as possibly being something of a research mathematician" after meeting Erdos in his 40s, although he did get degrees in mathematics when he was younger. He went on to publish over a hundred research papers: https://en.wikipedia.org/wiki/Richard_K._Guy#Mathematics

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...