I agree with ADay...start now. Very light and basic at first like ADay said about Mommies and Daddies. Slowly increasing towards the facts, like health class as they get older. Once they're mature enough, and you'll know when that is, start getting into the finer details of all that sex implies.
My oldest son is 15. We have a good relationship and I've been quite involved in our community...don't make the mistake that sex doesn't come up before they are teens. Without detail, my first realization of this was when he was 11. Though it didn't involve him, I was shocked and began more detailed discussion.
Today's society sells sex to a very young group. Many catch on through older siblings who are say...less than guided by their parents, and feel it "cool" to be like them. It's everywhere.
I feel the best thing is to be open with the child and let them know that no matter what the topic...you are there for them. Don't wait for her to ask you!