Fact Finding

http://www.superpages.com/supertips/age-to-start-kindergarten-by-state.html lists the kindergarten start dates by state. This list was probably posted within the past 5 years, but it gives a general idea. Your character was born in 1997, so you'd need to find a state with a suitable start-date and then find out what their policy was in 1997... or really? You could probably use current data or make it up in a vague way, because as the US has different dates for different states, people are entirely blase about classmates being older/younger, and likely readers won't know for sure either. (Also, boys are sometimes held back a year by their parents so they'll be more developmentally able to handle school, starting a year later.)