As everyone knows, the drinking age in America is 21. The age for pretty much everything else – voting, joining the army, signing contracts – is 18. Why is that?

Supposedly, it’s to stop young people from drinking. But who really believes that? It’s trivial for pretty much anyone under 21 to get alcohol. In my college, whenever you walked into a dorm room, the first thing you saw was all the bottles of vodka lining the shelf. Statistically, more than 85% of Americans drink alcohol before age 21.

And of the other 15%, many abstain after they turn 21 too. In fact, about 30% of Americans say they don’t drink – meaning that many must have tried alcohol when it was illegal, even though they don’t drink when it’s legal. At many colleges, police and school officials have pretty much given up trying to enforce this law, at least as it’s written. Even more than marijuana prohibition, the “drinking age” is an obvious failure.

But what if the real purpose of the “drinking age” isn’t to stop people from drinking? If you’re 18, the real effect of the law isn’t to stop you from having a beer (you can just ask a friend). The actual effect is to stop you from going to places – bars, nightclubs, parties – where alcohol is served. What if the real reason is to exclude college students from adult society?

At least in the US, alcohol’s most important function is social. We meet friends at bars. We have sex with strangers we encounter at bars. We hold parties at bars. We celebrate birthdays at bars, and so on. A huge number of social events involve alcohol. In fact, if someone often gets drunk alone, we don’t see this as healthy behavior – we might think the guy’s an alcoholic. So, the big effect of the drinking age is to exclude college students from social events that aren’t specifically designed for college students.

When I was in college, people occasionally talked about the “bubble”. If you were in college, all of your friends were college students. All of your activities were with student clubs. All of your parties were college parties. Basically, you only really got to know people who were of your own age, intelligence, and social class, and who had been filtered in many other ways (eg. work ethic, interest in “extracurricular activities”). If someone wasn’t a college student, being friends with them was weird. It Just Wasn’t Done.

Why would people want to create societies where everyone is the same? I don’t know. It’s an interesting question. But that people do seems to be empirically true. Fully half of the Ivy League schools (Dartmouth, Cornell, Princeton, Yale) are located in small “college towns”, which don’t really have much to offer other than the college. Of course, these colleges have also been getting more and more exclusive about who they admit – Yale, which loves to brag about its ethnic diversity, draws about half of its students from the wealthiest 2% of families. And before college, most children are highly segregated by age. 13-year-olds spend almost all of their time with other 13-year-olds, not with 12-year-olds or 14-year-olds.

That isn’t right. It isn’t normal. During prehistoric times, and Roman times, and medieval times, and industrial times – all the way up to the 20th century – everyone was friends with a variety of people, both older and younger than them. To quote Paul Graham:

“Teenage kids used to have a more active role in society. In pre-industrial times, they were all apprentices of one sort or another, whether in shops or on farms or even on warships. They weren’t left to create their own societies. They were junior members of adult societies.”

That’s what humans were designed for: the young learning from the old, and the old being helped by the young. That’s still how adult society works. Walk into any office, and what you’ll see is people of different ages talking, thinking, working and building together. If you proposed that everyone from 30 to 35 should be in one department, and everyone from 35 to 40 in another department, people would look at you like a lunatic. (Children do change more from year-to-year than adults do, but the percentage gap between 30 and 35 is the same as that between 10 and 12.)

The important thing isn’t alcohol. When I was at South by Southwest, at age 19, people were throwing bottles of beer at me (literally – I almost got a bad knock on the head!). But I couldn’t go to any of the events after 5 PM, because no one would let me in.

For some strange reason, very few people under 21 were there. And then we’re surprised when college students graduate, and 85% of them move back in with their parents. What’s the alternative, from their perspective? They never met anyone who could get them a job.