Supposedly in areas where poverty is higher, marriage historically would be allowed at an earlier age (marrying off a daughter alleviating the parents of another mouth to feed). How this could have made the AOC seven in Biden's state, I have no idea. I also don't understand the connection between marriage and sex outside marriage, but that is something I've heard.
At the opposite end, I used to hear the UN was trying to pressure countries to keep raising it to early 20's as the eventual goal.
These days I have lots of new beliefs. I think there's an outright assault on male-female intimacy generally, with a goal of destroying births, whether regionally or globally. One would think most child lovers would be open to hearing this idea. One would be wrong. It encourages venom from many quarters. Stuff I ponder and try to unravel nowadays.