I do agree there are good things on YouTube, but there are also horrific, definitely not age appropriate things, and there is not enough control to keep one flowing into the other. We watch YouTube as a family, but we're fortunate in that we all like the same content. But in my opinion, supervision is the only true way to make sure they're only seeing appropriate things.

Also, "their friends are doing it" is a terrible reason to let them do it, IMHO.