Don't get me wrong, I'm all for privacy. But between setting up the birthdate when creating my children's local account on their computers, and having to send a copy of their ID to every platform under the sun, I'd easily chose the former.
I'd even agree to a simple protocol (HTTP X-Over-18 / X-Over-21 headers?) to that.
You aren't setting up your childrens accounts. You're setting up your accounts to show that you're not a child. And suddenly, every single thing you use, from apps to websites, is gatekept behind an API that is controlled by the government. If checking age on social media is all it ever does, then sure, whatever. But that isn't all it will ever do. It will creep further and further, and the details you need to provide will increase, one shitty government term at a time. And then one day, they'll able able to decide that people in your country shouldn't be able to see safe sex information, or abortion information, and the framework to deny the whole country access is already there, and just one small tweak away from locking you out of information that is deemed inappropriate.