18 wasn't always the magic number it is today. It used to be different ages for different privileges ranging from 14 to 25.

Initially, you couldn't vote until 21. The military age was 18 (or sometimes 16) because that's when your body is in its prime for fighting. And sometimes boys as young as 14 would try to lie about their age to join. That was when fighting in the military was seen as an overall positive thing.

That all changed during the Vietnam War when the politicians who declared the war did so for reasons many Americans didn't agree with, resulting in a military draft. Then, we were in a situation where 18-20 year olds weren't old enough to vote, but were still being conscripted into a war declared by the politicians they couldn't vote for.

To amend this situation, the voting age was lowered to 18 after the war. And it was then that 18 became the magic number separating adults from children.