In the U.S., high school is 13+, depending on when the student was born.
Age of adulthood is 18.
I didn't realise that US schools have lower age, though - but no one differentiates between the young ones and the older ones even though the majority should be older than 15, right?
It makes about as much sense as it sounds like.
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.
In South Dakota you can get a permit at 14.