Ok, time for a little treasonous thinking.
The question: since when did "we" ever get the idea that 18 was the cutoff date for anything? You have to be 18 to vote. In some places you have to be 18 to drink. You have to be 18 to get married. You have to be 18 to buy guns and ammo.
But here are some more interesting things: You only have to be 16 to drive, at least in TX. You have to be 21 to buy a handgun and handgun ammo, at least in most of the stores I shop in. (Probably all, but I'm just going by the posted signs.) You have to be 25 before you get a "normal" insurance rate. But let's disregard the majority of this for the moment.
You have to be at least 18 to get married, at least in the US of A and most other places "civilized." There are some circumstances when you can get married before 18, but always with someone blessing the union--not God, but the parents, the court, or someone in like capacity. Now, to be more specific, since when did we get the idea that 18 was the minimum age to marry?
In other countries and historically, 18 was never the age. 15 or younger was common, and as young as 12 was not unheard of. In fact some marriages were set up in advance (read arranged) by the parents of the couple before they were even old enough to go to school. And yet today, we think, nay, require, that people be 18 years old to get married. Why?
Forget why for the moment. Let me ask another question: have we, as a society, created our own problems by imposing an arbitrary age limit on marriage? Consider: there is a perceived epidemic of child sex, teen pregnancies, single mothers, and, single parenthood families receiving state-subsidized support for their children. Why do we have this problem?
The answer is simple: "kids" are having sex. No, the quotes are not an accident. We classify anyone under 18 as a child, and anyone over 18 as an adult. But why are kids having sex? Not because of peer pressure or dares or etc., but because their urges have switched on. Biology has kicked in, and kids are trying to satisfy those urges. Is that wrong?
Joycelyn Elders didn't think so. Joycelyn Elders was Surgeon General under President Clinton. She saw what was going on, and she put forward a solution: Teach "kids" how to masturbate. Oh! how the public didn't like that. Dr. Elders was roundly and soundly derided by the US public, and she was shortly fired from her position. Was she wrong?
Churches, conservatives, special interest groups, and many others thought she was. Republicans love to trot out family values as one of their campaign planks, and they beat up on anyone who threatens so-called family values. People in general don't like talking about their sex lives, or lack thereof, and any activities to satisfy their sexual desires outside of a relationship. Churches... well, churches love to hold up an issue and vilify it by saying that it's against God's Word or commands.
Ok, for the sake of argument, let's say that masturbation is contrary to the Bible. Is "child" marriage? Oops... "Let's not go there!" is the mantra that many churches will take when asked this question. Why don't they like to talk about this? It's because of the highly-debated issue of how old Mary was when she gave birth to Jesus. Was she 18? Was she 15? Was she younger? "But that's a different culture and a different time!" protests the church. Granted, but were Mary and Joseph immune to the biological happenings of puberty? While we can't know for sure, I'll hazard an opinion and say they weren't.
"Foul!" cries the church! "Foul!" cry Republicans! "We're not in that country or that culture!" they protest. Fair enough, but, my original questions still stand: when did we figure out 18 was the threshold age for adulthood, and, if that assumption is faulty, have we created our own problems of childhood pregnancy, spread of VD, and all the other consequences that go with sex under 18?
From a purely engineering perspective, I have to say that 18 is an arbitrary number that, in effect, is meaningless. Consider: why is an inch an inch? Who decided that? Why is a day 24 hours instead of 20 hours, or a year 365.24 days instead of 400, or 300? Humans are very good at assigning an arbitrary number or label to something, and we do so for convenience.
18 and over is adult, 17 and under is child. That simplifies a lot of things. For the legal system, there is no thought given to what makes an adult an adult--if you're 17 or younger, you're a child. If you're 18 or over, you're an adult. Plain and simple. It requires no evaluation of adulthood. It also makes statisticians happy. They have a built-in category for kids: 17 and down. It's convenient to have labels and limits.
The inconvenient thing is that people don't conform to a standard set of rules. I've met kids at the age of 13 who were way more mature than I was, and I've got acquaintances 30+ years and older who still act like they are in college. To be sure, we ask "kids" to stay kids until they're 18, and then we expect them to be adults. Unfortunately, it doesn't work like that.
Kids break free of those lovely arbitrarily-set rules and limitations, but they can't break free from the laws of nature. When the sex drive kicks in, it's off to the races, and good luck keeping up. We ask them or expect them to exercise self-control. Some do, some don't, but are we expecting too much of them? Are we expecting too little of ourselves?
I've got more treason to spout, but not right now. Check back later for my continuation.