My first parental sex-education talk beyond "boys have a c*ck, girls have a pu$$y" came at the age of, I believe, seven, when my mom and I filled out my registration form for Girl Scout sleepaway camp together. One of the questions on the form was "has she started menstruating yet? If not, has she been told?," which my mother inevitably took as a cue to tell me. I don't remember her explanation very well, but I remember two points of confusion:
1) At first, for some reason I assumed that menstruation only happened for about a year during puberty, as some sort of kick-start thing that made you able to have babies. My mother cleared this up, at which point I was horrified by the concept that this happened once a month until you were fifty-ish. Gah!
2) I never mentioned to my mother, and thus she never cleared up for me, my irrational belief that menstruation involved expulsion of something that would look like a fetus, or at least obviously human in some way. I still don't know where this came from, but I attribute my current extreme mellowness about the topic on the fact that this particular nightmare did not come true.
The remainder of my sex-related conversation with my parents came at seventeen, right before I met my Internet Boyfriend for the first time at a sci-fi convention. The conversation consisted of "you know how to be safe, right?" "Right." "Okay, don't do anything stupid." Parenting!