It doesn’t have to (and shouldn’t) be the social media company gathering that information but a third party verification system that doesn’t save anything, just validates the age using a driver’s license/form of ID. We can do this (we do it for military and veterans so companies can give them discounts) in the US already.

Unless you have extremely strong laws around personal information, the financial incentive for any 3rd party to collect and sell that information will be too strong.

And if your 3rd party verifier is the government, then there will be equally strong pressure from law enforcement to collect that information.