My parents had "the big talk" with me when I was about nine or ten, but I wanted my daughter to always feel that she could ask me anything, so I have had lots of little talks with her over the years. That book, How your Body Works, has lots of other topics besides how babies are made, like the heart and circulatory system and why your body needs sleep, for example. I think I bought it for her when she was four and we have looked at all the different things in it together. Now more recently she reads it alone, which is fine with me too.