Perhaps he thinks a human "becomes human" when it is self-sufficient.
...around age 2 maybe when the little buggers can run around, feed themselves, etc.
A newborn infant will die without intervention (feeding and "upkeep").
My point is that ontologically it doesn't make sense, literally, to speak of when a human becomes a human. It wouldn't matter what object we were talking about. It would be just as self-contradictory to speak of an orange becoming an orange, because an orange cannot be an orange and not be an orange at the same time.
Cordially,