The typical student starting college in the US is 17 or 18, depending on when during the year their birthday falls. You can get the occassional student who was accelerated through high school and starts at 15 or 16 (I assume you mean actually matriculated as a college student, not those wiz-kids who start taking college classes at 13, but are still technically in secondary school half the day), and of course there are people who take time off after high school that can start college at any age, or who have been held back for some reason in school, so are a bit older at high school graduation (it doesn't always mean someone was flunked out...I've known of people who were seriously ill at some time during their secondary school years, so had to re-do a year of school because they missed too much).