Some states such as Virginia (although with an exception if the person has a high school diploma or equivalent):

https://www.vccs.edu/application-information/

To qualify for general admission to a college, you must have a high school diploma, its equivalent, or be 18 years of age or older.

Unclear whether that is statewide or just a requirement of this state institution. I suspect many of the ones that do have this age requirement also have the diploma exception.