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I've never hidden anything from my DDs, and have always answered their questions factually. However, DD1 (9yo) and I have been talking in more detail about puberty and so on for the last few months, as I think she's not far off. I don't know the book that you've linked but I've been using this one with DD1, and it's good for her age group.
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.
I've used the what's happening to me books for all three of my dc's at around 10. Great books, dd's often refer back to it months later. I think 10 is about right although I've always answered any questions honestly, so no mention of storks.