They didn't graduate high school or college on time
Maybe they repeated a year at boarding school because their parents told them it's easier to get into college during sophomore year. Or maaaayyybee their parents were concealing the fact that they didn't think they were mature enough to leave the nest yet. They obviously took a gap year, and you know this because they will somehow find a way to shoehorn it in (unless it's a PG year because who wants to spend ANOTHER year in high school when their friends are doing body shots at frat parties?). And perhaps they partied a little too hard because they lack discipline so they had to take a leave of absence to get their shit together. Maybe their parents sent them on a NOLS trip to give them perspective or maybe they were in rehab for their debilitating Adderall problem. Whatever the case, you'll notice their "class of" year will not correspond with their age.
[Photo via @trinitycollege]