You stop being a minor at 18 for legal purposes, but there are still random restrictions on what you can do (like drinking or buying handguns) until 21.
Yeah, at 18 you don't have enough sense to drink alcohol responsibly, but you are allowed to vote. This shows how little this country thinks of voting.
I think it has to do with 18 also being the age you can be drafted and sent to war. Theoretically, it keeps the politicians who send people to war answerable to those people.
It's also worth noting that until recently the drinking age varied from state to state. I grew up in New York and it was 19.
The uniform 21 age didn't come about until pressure groups like MADD went after the politicians. I believe that the feds ended up tying a 21 mandate to highway funding. (The same trick that's used to keep the public schools in line.)
And since we're on driving, the driving age varies from state to state, as well. It's as low as 16 in South Dakota. Most states it's 18, but lowered in "hardship" cases, like farm families where the kids might need to drive a tractor across a public road to get from field to field.
