I guess it's carrying on the nuclear family thing where kids are supposed to be booted out on their own when they hit 18 and "find themselves" before settling down.

With a larger family like on family farms and stuff kids can become financially stable and responsible enough to raise their own by the time the rest of us are snorting adderall in college. That'd go a long way toward reversing our failing birthrates though, so gotta make sure we worry more about becoming middle-class and buying our own McMansion in the suburbs

It’s also the age the average person graduates college.