In answer to your first question, what do we do. We regulate the platforms, not the users.
There are loads of things that can be done to reduce online harm, that don't just say "OK, you're over this age, you can experience online harm now".
Platforms can become legally responsible for content, they can be compelled to be transparent about their feed algorithms, enable monitoring via public APIs. We can ban infinite scroll. There are so many policy levers that can be pulled, without getting the ban-hammer out for people of an arbitrary age, and without forcing the rest of us to hand over our IDs to platforms.