It will vary from state to state, depending upon the cut-off date to start Kindergarten.
DD was born in Sept of 1983. Cut-off date in Fl is Sept 1st (child had to be 5 or older by that date) So DD didn't start K until Sept 1989
Niece was born in Dec of 1984. Cut-off date for MA (at that time) was Dec 31st, so she was able to start K in Sept, 1989, the same time as DD, even though DD is 15 months older. Basically, DD has always been one of the oldest children in her class, while DN has been the youngest in hers.
They both graduated high school in the same month, and will do the same with college.