Unless things have changes since I was a teenage boy (and they might have, that was 30 years ago!), teenage girls are interested in a very, very small subset of available teenage boys while being visibly, publicly disgusted by the rest.

"interested" or "has access to"? most teenagers meet other teenagers in or through school, where they are rigidly separated by year in most activities. it's not surprising they date people exactly their age when that is the majority of people they encounter day-to-day. by college, it seemed to me that the guy was often at least one to two years older in hetero couples. of course, same year was not uncommon, but I can't remember a single case where the woman was more than one year older than the man.

Sure, that’s believable as a casual reason.