IIRC, The Mighty Offspring has known from a very young age that he came from the hospital. Later, as it occurred to him to wonder why he was at the hospital, he was told that he was in my tummy and needed to be got out – c-sections were explained, although we did say that wasn’t the normal way for babies to come out. It took a while for him to wonder how babies normally come out, so that was explained, too. So on and so forth, mostly led by his own questions and theories.
He is, however, an enquiring child and much given to theorising about the world, so it’s been relatively easy to be guided by him. He’s been more or less fully clued up since the age of 8, including knowing about periods. It hasn’t destroyed his innocence, just satisfied his curiosity.
He’s 11 and still thinks girls have cooties.