Genuine question, cause I don't live in the US, is starting college at 17 unusual? Over here half of the cohort finishing high school are 17, so I expect a similar proportion starting college here are 17.

Personally I had a 2 year gap between high school and university, so I went from being one of the youngest in my year to one of the oldest,which was kinda nice. Plus the time away taught me some self reliance so college was pretty easy to adjust to.

Canadian here, but I think the US follows the same age-grade mapping. Kids usually enter Kindergarten in September of the calendar year in which they turn 5, i.e. those born January-August are 5 years old and those born September-December are still 4 years old. After that, grades 1-12 follow with a majority of post-secondary-education-bound students entering a few months later.

So yes, in the modal case (advancing with the cohort) a student entering college will start at 17 years old if their birthday is late in the year.

In most US areas, grade cutoff is around August, so before college starts. Most students start college at 18.