My kids are still young but I just say things like how your body changes as you get to be a teenager...you grow more hair all over your body, especially around your groin area, and your body starts stinking so you need to wear deodorant, you may get pimples, etc. I don't do big talks bc it's awkward I just weave it into conversions when we're driving or cooking or whenever. The earlier, the better. I think kids get so embarrassed when they are older.