Well, I believe that it seriously depends on where you live. I live in a really good neighborhood and go to a school where sexual activity goes almost unheard of (drugs, crime, bullying is also uncommon). Many of my friends are not even allowed to date until college or later; "[we] need to focus on academics." If you live in a neighborhood like this, I would probably give "the talk" in your child's later middle-school years or maybe in high-school years. I would stick to However, I understand that many are not as fortunate as I: there are myriad communities where teenage pregnancies are common. In that case, I would probably explain everything by middle-school. If you have a daughter, I might give the talk when she gets her first period. Also, I would probably sneak in some peer-pressure topics as well.
I am still a high-schooler, but this is what I would do when I have children. Good luck! :)