>>512161849The thing is not biology but law, also education
18 is kind of arbitrary but it coincides with the age most teenagers have already finished their basic education.
At 18 the law and society assumes you were taught enough to understand the consequences of your actions, and thus you are also given the right and duty to vote. Also, you are not under the legal protection of your parents anymore and also you are not under any legal obligation with them.
A 18 can (if laws permit it) charge for sex because a 18yo is expected to understand the consequences of said action, a 16 yo is not expected because the education is not complete yet. And no, it doesn't requires that the teenager to have actually learned anything or even passed school, but that it was given this knowledge.
This is just how things are, anon.
Also another thing related, as 16yo are still under their parents tutelage and responsibility, one could think that it is the parents who are forcing their own children to sell their body for money. A kind of abuse.
Look, smarter people than (you) and (me) have made these laws and they do make some sense. Sure they may look arbitrary but they do make sense when you see them in the context of a majority of age and the responsibilities that gives to the individual and the state mandate education the kids have to recieve. Heck, to the point that not giving children this education is considered an abuse and thus a crime.