Teenagers are stupid, every last one of them including us back when we were teenagers. Mistakes are made, dumb things are said, and the internet makes it staggeringly easy for them to reach a massive audience. Kids shouldn't have to carry a dumb comment or a scantily clad photo into their adult lives, which Facebook makes very easy.
The depth and breadth of the issues and content discussed on Facebook is so vast today, it's simply not appropriate for someone under the age of 16 years old. Certainly, there are ways to mitigate this issue to a degree with privacy and security settings, however, it's simply overwhelming to do this effectively.