Nothing, in the very early grades. If we did a really good job in the first 10 or 12 years of children's lives teaching them about abstinence, as well as about honesty and integrity and responsibility and how to make good decisions, we would not have to be talking to them at 15 about not getting engaged in sex.
But we haven't done that. Mothers have been teaching abstinence, schools have been teaching abstinence, preachers have been preaching abstinence for years. Yet more than three million teens get STDs every year, and we still have the highest teen pregnancy, abortion, and birth rates in the industrialized world. But we seem to feel that we don't need to educate our children about their sexuality. That makes absolutely no sense. We all know the vows of abstinence break far more easily than latex condoms.
Teens need a comprehensive sexuality program that gives them all the information they need to become empowered and responsible for preventing pregnancy and disease. We have to stop trying to legislate morals and instead teach responsibility. Abstinence-only does not do that. You can't be responsible if you don't have the information.
I have no problem leaving it to parents, if you have parents who can and will do it. But we have many dysfunctional parents -- some on drugs, some into alcohol, some who are stressed out, and some who just don't know how to talk to their children about sex. Then the responsibility belongs to the community. And since the only place we've got access to every child is in school, we need to use our schools to teach about sexuality. We don't depend on parents to teach math and English and science and geography. So why should we depend on parents to teach children all of their social and behavioral skills?