At what grade level should sex education be introduced in the schools?

Personally before HS: I feel personally sexual education is being taught in school much too early. I feel sex ed should be taught prior to the time when most youngsters would engage in sexual activity, not any youngsters would engage. For those parents who feel strongly about teaching their children sooner, they should teach them at home. For those parents who feel 8th grade is too soon, then sign the note to opt out!