Require by law that everyone on social media authenticates their age (maybe also their names, this would be good way to get rid of bots as well). This can be done with government ID or with via your bank or even with requiring social security number when creating an account. Or government can just make a new digital ID system that you have to go and register at whatever place is suitable for this and you can use that ID for anything and everything you need.

And if that doesn't seem to work just fine social media companies say a million dollars per minor per day that minor uses the service. You see enforcement go up fast AF.

Banning all social media is even simpler.