Ultimately, it's a comedy
Sometimes, age ratings make me think that we are quick to think that things like sex and swearing are important not to model for teens, who probably more than any other group needs to laugh and see the absurdity of things, rather than be depressed beyond measure at endless violence, cruelty, incompetence, greed, and apathy portrayed in expected and tasteful ways. Joan of Arc was executed at 14, I don't think it was comedy, probably it the opposite that was the forerunner there for everybody. It also reminds me of the studies where they see at what age children understand when something isn't real and isn't supposed to be real, it's before kindergarten. It's pretty clear in this movie who the bad guy is, and the faults that the characters have, that we all have, maybe in not such funny ways. There are probably few 14 year olds in America who don't realize that they are making fun of the very real tendency in the fashion world to reward being thin, please don't worry that making fun of a bad trend will be interpreted by kids as something to emulate, quite the opposite in my experience.