According to Songfacts, Fifteen is about Taylor Swift's best friend Abigail.
Swift told The Los Angeles Times that for most of their time in high school, the two friends thought of themselves as ugly ducklings. She explained: "We kind of came to the conclusion in ninth grade that we were never going to be popular, so we should just stick together and have fun and not take ourselves too seriously. That's why I had so much fun in high school, because I didn't focus too much on the fact that I wasn't really in the clique." .