How about Raymond Smullyan? According to his autobiography[1], 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?
[1] Raymond Smullyan, Emlékek, történetek, paradoxonok. TyopTeX, 2004, original title "Some Interesting Memories. A Paradoxical Life".
