"The exact identity of Swift’s first high-school boyfriend is unclear, but whether he was Dunlap or Borello, the story is the same: He was a senior, Swift was a freshman, and like many high-school couples, they broke up when he went away to college. [It's hard] to say [where they are at in life now]. Borello seems to have gone to ground, while Dunlap appears to be living a relatively normal life as the most-Googled project manager in the greater Nashville area." -vulture.com
It’s been speculated that this song was written for Drew/Brandon when he was a senior dating Taylor who was a freshman at the time, and how she wanted him to remember their times together before he went off to college
Although a possible stretch, this song mentions Taylor being in the passenger seat of a boys car-meaning most likely she was younger than him and he was older- leading to the guy possibly being Drew/Brandon
Fifteen is the usual age for an underclassman and/or freshman in high school, alluding to the age Taylor was when writing about the boy who she "swore [she] was gonna marry him someday, but [she] realized some bigger dreams of [hers]."