Stephen Chbosky wrote the book that the movie is based on. He also wrote the screenplay and directed the movie.
In Entertainment Weekly, Emma Watson reported that she took the role because Stephen Chbosky told her that "Not only is this going to be one of the most important parts you play, you're also going to have the summer of your life and meet some of your best friends." She also reported that the claim came true.
According to an interview with Stephen Chbosky, "watching people like Emma Watson, Logan Lerman, Ezra Miller, and Mae Whitman who never had prom, never really had a proper graduation, because they were always working, and watching those kids get to have a high school experience that we all take for granted, was very special."
In an interview with LAYouth.com, Stephen Chbosky said he wrote the book for personal reasons, but reading the book's customer reviews on Amazon.com made him realize that many people related to it.
It's not mentioned in the movie specifically, but Charlie is close in age to Sam and Patrick. Charlie's birthday is December 24th, and his cake has 16 candles. A December birthday would make him an older freshman, but the book mentions that Charlie was held back a year due to emotional problems. When Charlie turned 16, most of his Senior friends were probably 17.