Ages 2 to 3: The right words for private body parts, such as "penis" and "vagina". It is for the parents to decide the proper language to use with their kids.

Ages 3 to 4: Where babies come from. But they won't understand all the details of reproduction -- so a simple "Mom has a uterus inside her tummy, where you lived until you were big enough to be born" is fine.

Ages 4 to 5: How a baby is born. Stick with the literal response: "When you were ready to be born, the uterus pushed you out through Mommy's vagina."

Ages 5 to 6: A general idea of how babies are made. ("Mom and Dad made you.") Or if your child demands more details: "A tiny cell inside Dad called a sperm joined together with a tiny cell inside Mom called an egg."

Ages 6 to 7: A basic understanding of intercourse. You can say, "Nature [or God] created male and female bodies to fit together like puzzle pieces. Explain what you think about sex and relationships. For instance: "Sex is one of the ways people show love for each other."

Ages 8 to 9: That sex is important, which your child has probably picked up from the media and her peers

Ages 9 to 11: Which changes happen during puberty. Also be ready to discuss sex-related topics your child sees in the news.

Age 12: By now, kids are formulating their own values, so check in every so often to provide a better context for the information your child's getting. But avoid overkill or you'll be tuned out. 



Source Talking With Your Young Child About Sex (Copyright © 2000 American Academy of Pediatrics)