Adults are supposed to have acquired enough life experience to make more reasonable decisions than children.

Having said that, we also expect adults to make all kinds of bad decisions so they too are prohibited from many things, like shooting smack or freebasing meth.

I think there's a good case to be made not for banning kids from social media, but for banning everyone. In that case, it'd be easier to do it from the supply side than the demand side.