It's clear that Auden feels older than her years — and that her parents expect that of her. In the adaptation, it's implied that this is a side-effect of Auden's parents' divorce when she was a young child. In the book, though, there's a secondary reason: Auden's free-spirited older brother, Hollis, who's aimlessly travelling abroad for most of the novel. His tendency to shirk responsibility and act out, especially when they were children, forced Auden to be more of a "little adult," she explains in the book.
Hollis's gift of a tacky picture frame, featuring the words "The Best of Times," is partially what convinces Auden to take the leap and spend the summer in Colby; she realises she doesn't know what to put in the frame, because she hasn't had any experiences that fit the label. While Hollis didn't make the movie cut, the picture frame does get a cameo at the end of the film.