It is wellknown that many great mathematicians were prodigies.
Were there any great mathematicians who started off later in life?
It is wellknown that many great mathematicians were prodigies. Were there any great mathematicians who started off later in life? 

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In the early 90's, I had a colleague who had been a professional tennis player before going back to a PhD thesis in mathematics. I'll hide his name since he is now in the middle of his career. 


Dennis Sullivan who won the 2010 Wolf Prize comments on this in his own words in the magazine published by the New York Academy of Sciences: http://www.nyas.org/Publications/Detail.aspx?cid=14047af09f26481c9b509434130c89db 


I don't know if Jean van Heijenoort counts as great, but his life story is amazing. 

