if the dude says he's 18, we don't really have any reason to not believe him. i finished highschool when i was 18, but there was a girl in my class who was 21, because she was held back a couple years. i have friends who only finished highschool at 19.
it also depends at what age you start school. if you start at 6, you'll be finishing highschool at 18, but if you're 7 when you go to first grade, you'll be 18 in your last year and finish highschool at 19