Questions about how this is going to be implemented and enforced from a technical and legal perspective are missing the point/benefit: this is about empowering parents and collectively changing behaviours.
"It's against the law so no you can't" isn't going to work with EVERY 14 year old. But it will work for many and hopefully that's enough.
I actually see this as potentially damaging to society. "It's against the law for you to use any website that lets you look at cat pictures and make any contact with anybody else" is so silly that kids are going to see right through that, and rightly not care about following it. So they're going to have less respect for the rule of law generally...
I'm very big on compentent laws, but also on just not having silly laws. It devalues the whole system...
(I would also wonder how many 14-year olds you know if you think this would work for many, but also I suppose that could be a cultural difference)