I went to a very good public school in the US, and we didn't have sex education until 4th grade, and even then I think we were a bit immature for it at the time. They were still only doing the _very_ basics and I remember the class not being well behaved during it. I think sex education any younger than that is totally unnecessary and kind of inappropriate.
I find it pretty weird that American politics has become really obsessed with the sexual topics around little kids lately.