It also ignores social pressure entirely. If everyone in your class is on some social network and your parents parent and you aren't then you are socially outcast from a major part of your peer socialisation.

If only 50% of parents enforce the rules suddenly half your class isn't there and it doesn't become such a big deal to be missing out and it's less appealing for the kids who are allowed still.

I'd prefer it to be voluntarily organised like https://www.waituntil8th.org/ but even a bad solution is going to have a major improvement on society even if social media is less private due to the ID issue. It's not like you don't share everything with these companies anyhow. I'm pretty confident that if you are a regular user of any large social network today they could identify you 100% of the time already even without your ID.