Personally I don’t have an issue with not allowing children on social media, but with how it’s going to get enforced. Do you trust your government with ID data that’s immediately linked to everything you say or do on a social media website? And I’d be surprised if the social media companies don’t get access to that ID data, too. You could implement age checks in a privacy-friendly way (I mean one that actually works, not just “click yes if you’re 18”), but they’re not going to.