The Motion Picture Association of America rates films, iTunes marks music downloads as "explicit" or " clean," and the Entertainment Standards Review Board decides if a video game should be rated "M" for mature, "T" for teen, or "E" for everybody. Some people wonder why the book industry doesn't create an easily-recognized ratings system for people concerned about the age-appropriateness of books. The answer is easy — books don't need a regulating board like the MPAA or the ESRB because information about a book's content is already abundant and accessible (unlike the content of other genres of entertainment.) The age-level suggested by a publisher is usually on the copyright page, as well as keywords that provide even more insight into the subject matter. No other source of story or entertainment is as widely reviewed by so many reliable sources. An industry-wide board overseeing a multi-tiered ratings sytem would be a step backwards from the wealth of information that the publishing industry already provides for any literate parent — or teen — who wants to know what's in a book.
A problem, however, exists when the digital, linguistic, or literacy divide prevents a parent — often an immigrant parent — from accessing that information. That might be why savvy librarians use a simple colored sticker system to classify the age-appropriateness and/or content of books. It might also be for the young reader's sake — many an immigrant teen, desperate for a reprieve from the constant push of "edgy" pop culture, takes refuge in an old-fashioned, wholesome story. A library-based sticker system provides quick info for any reader seeking safety on the fire escape.