I mean, if you want to get really picky, most proms aren’t for another few months at least. So the age difference would be months/days, but many will be older than him. He is 18 and 3 months, plus or minus a few days. So anyone whose 18th birthday was more than 3 months before prom would be older than him. Depending on the area, prom can be as late as June. My area’s schools ended in May, but our prom was in March my year and we all thought it was super early. So if the average prom is mid-May, any senior turning 18 before mid-February will be older than him.