Well I do think and believe sex educational classes should be taught as early as the fifth grade. The more you educate young students you can make them aware about the dangers of contracting a sexual transmitted disease.
at 5th grade it should not be talks of how to have sex, like "sex is when a man inserts his genitals inside a womans genitals, and when they do it, it feels good and is natural" as that just entices people to be more curious and want to experiment/try it
at 5th grade it should be about preparing for the biological changes of puberty, how hormones are going to affect their feelings, emotions and how they may react in situations, how they may perceive and see the opposite gender, and how hormones can overwhelm their decision making process.. where they need to learn to think before they act and learn to respect each other. whereby its not just about sex but also about avoiding the rebellious teen stages of acting out and going against good life choices
teaching them about relationships and consent
then introducing more detail as time goes on.. basically an actual planned curriculum suitable for the age group. and not "thrusting" the word sex into them