COPA already effectively does this, but kids just lie about their age. Making FB and company liable for those cases where parents decide to let their preteens go wild on social media seems insane.
True, but it only takes a single country anywhere in the world to keep the internet free. And by keeping the net free, they'll get all the money, so they're is a strong incentive to do so.
In the event there are no countries left there are still options:
1. Hidden services
2. Decentralized
3. Just create a pool mechanism where you contribute your private key for whatever gov Auth system exists, then use a random id from the pool among all sign-ups everytime you sign up for a service.