Virginia state law is that "Children who will be six years old on or before September 30, must, by law, attend school." So people starting 9th grade should be either 13 or 14 - a fifteen-year-old starting 9th grade either had some sort of special circumstances that led to delayed enrollment, started school in a state/district that had a different cutoff date, or was held back.

(On the other hand, some senior high schools still start with 10th grade, in which case everyone not born in September would start at age 15.)

(I think, if I did the math right.)