Talking with your kids about sex is happening earlier than it did when I was a child, because life is a lot different and kids are growing up faster these days. We like to start when they are eight years old because that is when they are old enough to understand, yet not so old as to get too embarrassed by the subject. And it means we, hopefully, beat their peers and the schools in educating them, so we can give it the treatment we want our kids to hear. It is important to educate your kids about this subject so they are protected from wrong information and from peer pressure to do things they are not ready for....and to protect them from teen pregnancy.
Another thing to note, is that I have a friend who works at Washington State University doing a study on what works when talking to kids about sex and prevention of teen pregnancy. She said that they found that if the kids were made to feel guilty or bad about it, then it was LESS successful at preventing problems, than if they were taught the subject in a matter of fact way that didn't make them feel bad, or in a way that helped them feel good about self control.
There are two books we use when broaching this subject with our kids. And we chose them for the fact that they make the kids feel good about the whole subject and self-control.
The first is called, "How to talk to your Child about Sex" by Linda and Richard Eyre. They also have a great parenting website.
And the second book is called, "Where did I come From?" by Peter Mayle.
And later when kids are going into puberty, we like to use this book, called "What is happening to me?" by Peter Mayle. (although we do like to edit a couple of these pages with our own slant on things)
We usually take our kids on a parent/child date and eat a nice dinner and then talk about things afterward. My kids love it and look forward to their "Great to be Eight" date! Hope it works for you as well as it did for us!