I teach 4-6th graders a class on sex education called "Our Whole Lives". We use the book "It's Perfectly Normal" with the idea that the parents will read it too so they can talk with their kids and answer any questions. I've found quite a spectrum in terms of how kids react, from "oh gross" to "so that's what it looks like" but all kids seem to appreciate hearing the facts from a trusted parent or teacher. These days, with puberty starting so early especially for girls, I think it's a good idea to cover the basics around 10. You can then go back and spend more time on some details as you DC grows and/or shows interest.
