Our oldest boy (almost 11) started asking some pointed questions before he was 9 years old, and we basically spelled it out for him, using proper terminology, and told him that if he heard anything in school or wherever that he wanted to know more about, to come and ask us. Neither my wife or I have any major embarrasment issues with covering the basics, so it wasn't very difficult. Of course, one unintended consequence, was that he turned around, and started passing the info along to his younger brother, who now, at age almost seven, is fully versed on the technicals, thanks to the curiosity unleashed by his brother.
I think when we were kids, it was easier for our parents to keep their heads in the sand, we weren't exposed to nearly the same amount of sex imagery and conversation in society. But I do feel strongly that kids need to hear it from someone that they can trust, and never be made to feel that there are off limits topics.
Fish