How about Raymond Smullyan? According to his autobiography, he has published his first mathematical article at the age of 35, to which Marvin Minsky has reacted by saying Ray has decided to become a child prodigy at the age of 35. Does this count as starting off late in life?
 Raymond Smullyan, Emlékek, történetek, paradoxonok. TyopTeX, 2004, original title "Some Interesting Memories. A Paradoxical Life".