DD1 is 6 and knows about conception and pregnancy, that she will grow hair and boobs when she is older etc, becuase she has a younger sibling and I have hair and boobs. She also knows about how a lesbian or gay couple might have a baby, because she asked. I just answer questions, although I will be pushing it a bit more over the next couple of years - I started growing boobs at about 8, and periods at 11, and apparently it often gets younger through the generations.