What age are people when they're seniors in high school in the states?
Normally 17-18 but it's possible to be one way earlier if you're smart enough.
It doesn't matter how smart you are, most smart people actually try to stay there as long as possible (from my experience). What matters is the age at which you went to school, I started when I was 5, so I will do my finals 17 years old. Most others are 18 or 19.
EDIT: That said, I am not sure how it is in US or in normal schools.
Our definitions of "smart" are very different it seems...
I graduated 4 years early and subsequently had a free ride to any university I wanted, and I already had 4 years in uni and was in med school by the time everyone else was graduating high school.
If these "smart" people are staying there as long as possible, believe me, they aren't nearly as smart as you think they are. Just sayin'