For those jumping on the educators saying 'what do they know', I pose the same question to you all because you all seemingly know nothing too.
It's got nothing to do with addiction, changes in behaviour, what teachers think is best, or whatever other cock-a-hope you want to make up. There's a reason for it, and it's simple. Children aren't allowed accounts on such sites until they are 13 because of the data those sites hold. There's nothing to stop them browsing those sites, but they are not legally allowed an account until that age. I'm not sure the rules of this site, but I'm sure similar exists (or it should) that you have to be 13+ to sign up. It's for no other reason than it's illegal to hold information of children below that age.