I am mostly guided by DS as to what he is interested in knowing. He knows how babies come out (both vaginally and c/section). He has seen animals mating (but hasn't yet twigged what humans may do). He understands periods and knows what tampons are, and what tampon bins are for, and he knows the proper names for the body parts. So that is about what we are up to. He is 5. I am guessing he will ask more when he is interested in more and I will just try and answer honestly.