My DD asked very specific questions at 3, because I was pregnant with her brother. They went on and on (she really wanted to know), so we answered. The child psych view on this is basically that you respond to their questions at any early age based on what they seem to want to know, but if they do not know "how it works" by around age 6, it's time to give them some basics. Otherwise, they ARE going to get it from their friends at school, and don't you want to be their first teacher? Otherwwise they may really get it wrong for some time. I knew kids who thought you could get pregnant by sitting on the toilet in the boys' bathroom.
I'm talking about "where do babies come from" here. Not rape and abortion. I don't know what the thinking is on that. We have not covered rape and abortion, though we have discussed birth control (quite specifically, thanks to a gas station condom machine). Both kids know about periods and so on because they have asked about tampons, etc. I don't consider this info shocking.
Oh, and DS asked at 4 ("How does the baby get inside the uteus?" or something) but I'm not sure he remembers. It was a short conversation because he didn't seem to be as intensely curious as DD. Again, I was following his lead, since he was prtety little.
If the subject is horribly awkward to you, there are some nice books. We got DD It's Not the Stork when she was little, and there are more in that series.
Research on this endorses the key importance of being open with your kids from childhood up if you want them to share their concerns about sex and dating with you as they grow up. I would feel uncomfortable dancing around these subjects with my kids, especially since they're so bright and so perceptive. Especially with my highly stubborn autodidact eldest, I'd assume she'd find the info elsewhere. I don't want to give the impression that sex is something I don't want to talk about with them or that they can't bring up with me, or that it is a subject that "we don't discuss."
