In most places, the legal age is 17 or 18, so the 17 is the base they go off. Movie and game ratings follow this too. The one that Apple uses resembles that of games.
I remember seeing somewhere that any app that has a webview automatically gets the 17+ rating. I haven't used the Facebook app in about three years now, but I think it opens stuff in Safari, yes?
Facebook doesn't have objectionable material to begin with. The people you follow are the ones who make objectionable material, and most of the little kids running around with it on their devices are using it with other little kids and their family.