My personal opinion, it should be a uniform 16. 15 is too young, 12 is absurd (see those Mexican states), and 9 (Yemen) is plain evil.
16 was what I grew up with in California...but that was when voting age was still at 21.
The general mantra was, “a drivers license is a learners permit; but 15 will get you 20”. Made things simpler.
As to birth control, there were two basic methods: abstinence, and her daddy’s shotgun.