"You've only ever done that twice though right?" is a common reponse that other parents tell me their kids ask after they have "the talk" with them! That is hilarious! But "The talk" ....mmm, not so hilarious.
I know there's varying opinions on when to tell them. Some say when they ask questions, some say figure when they hit a certain age. I realize every child is different and that girls, especially, start to develop earlier than when I was their age (Lord, I hope she doesn't get her period until she's about 25 at the very least) and in that respect, you don't want them to be unprepared.