I have three friends who started college more than a year younger.
A senior who is now 20 (turns 21 in July), a junior who is now 19 (turns 20 in September), an a sophomore who is now 16 (turns 17 in January).
I have another friend who attended another college for freshman year, dropped out early on, took the rest of the academic year off and reapplied to many colleges including Duke- starting college at Duke at 19 AND turned 20 the September of his freshman year.
It's not so rare to not start college at 18.
I almost skipped a year myself (in hind sight, it would have sucked since I would have only been able to drink legally for two months, now add a year,
) but my parents thought it was better that I whored out the APs in my school- so that's what I did instead..
It seems to be especially common for internationals who didn't go through the American school system.