If you wait until your kid is a preteen, you’ve waited too long.
Age-appropriate honesty about sex should be a constant part of parenting. It starts when your child is very young and focuses on teaching children that they are in charge of their own bodies and have a say in who is allowed to hug and kiss them. It starts when you teach your young child that parts of their bodies are considered private and they have a right to insist that others respect their privacy. It means modeling a healthy adult relationship based on mutual respect and love, not associating sexual feelings or questions with shame or embarrassment, and instilling enough trust in your children to let them come to you with any question about their bodies.
By the preteen years, a child should know how babies are made, how to respond to inappropriate attention from both adults and peers, what to expect from puberty, and who they can comfortably talk to about all of these things. Because they are still children, however, parents have a responsibility to stay involved in the child’s life and limit exposure to TV and internet. Talk about what you do see on TV or online, and give kids the tools to make good decisions. They won’t always make good decisions, of course, but ideally a parent will help the child learn from mistakes.
By upper middle school (if not sooner), children should know how to protect themselves from unwanted pregnancy and STD. Parents can’t trust schools to impart this information, but should do it themselves.
I have an 11 year old daughter, and believe me, it’s not easy to look into your own kid’s big blue eyes and use words like “semen” or “erection” or “AIDS.” It’s particularly difficult when your own parents didn’t talk about things like that and you have no basis for comparison. It is extremely important, however, and so I make a big effort to respond honestly to her questions in a casual and non-judgmental way. It actually gets easier as time goes on. Last night we had a conversation about pubic hair and I didn’t even blush.