If you're talking to them about sex and they're teenagers, you've missed the boat. Kids are getting pregnant at 12 at an unbelievable rate. If I hadn't seen it with my own eyes I wouldn't have believed it. They know at 8 & 9 what the deal is, scary stuff
In this day and age the earlier the better. There is so much out there and not all of it good. Plus it depends on the child and the circumstances. As difficult as it will be, be honest and don't try and hide anything. Try not to over-react if they ask a shocker, might just be something they heard and are curious. Good Luck!!
Before their teen years... right when you start seeing signs of impending puberty. I know that it seems like you're taking away some of their innocence... but wouldn't you rather they learned it from you than in the gutter the way most of us did?