I'm asking because both my girls know the ALL of it already. The moment they started asking questions was the moment I took the opportunity to explain everything. In the beginning I don't really feel they understood much. DD1 was 4 when our neighbor was pregnant and that of course got the ball rolling. Now that she's getting older she asks more direct questions and we talk about it. DD2 listens and asks questions as well. They don't giggle they don't seem embarrassed. They seem to think at this point it's just life.
Tonight we talked about menstrual cycles. DD1 is very interested in it and wanted me to remind her why it happens. So 30 minutes later DD1 asks " Do you have to wear tampons, or can you wear a diaper. I think a diaper would be and excellent choice" DD2 asked if there was something wrong with the sperm or the egg? I didn't know what she meant at first but then she reminded me that we lost 3 babies this year. So was it the sperm or the eggs that were bad? Of course at this point we're not sure what happened but she has been listening and really understands quite a bit.
I do tell them that it's not okay to discuss these things with their friends, their parents may not feel it's appropriate for them to know yet. They're pretty good about it so far. However I was lucky to have a friend that told me everything I needed to know when I was a teenager since my mother was not about to say one word about it. She did give me a book but even that left me more confused.