Is your comment supposed to be satire? No one has ever said "When did they become a genius?" about someone who is considered a genius. I think, at least colloquially, genius is considered to be innate.

By understanding how a thing works, the more power one has to change it. Therefor, the better one understands their own intelligence, the more power they have to change their intelligence. However, being able to tease apart intelligence requires a high prerequisite intelligence, muddying the water between innate and not. In whole, I do not think intelligence is exclusively nature or nurture.

Asking someone, "When about do you think you became a genius?" is a great question. I might have to steal that one next time I bump into a genius offline.