I actually did a paper on this back in college [can't remember for which class though :lol:], found out the parents who usually made their kids opt out of school sex ed never taught their kids about it at home. To them, ignorance was bliss, didn't matter if they traumatized their kids as a result either. The parents tended to be more religious & had little advanced education themselves.
Their kids tended to be more likely to become teenage parents than the kids who were taught about their bodies & sex ed.
BTW, my mom sat me down & taught me about menses & sex when I was 9, just before I left for summer camp. She didn't want me to freak out in case I got my period, like she did when she started. Her mom was part of the unmentionable generation.
Don't think 9 is too early, my SIL, the family practice doc, sees lots of young girls who start menstruating at 9 or 10. It has become much more common than it was a generation or two ago.