>It's not that anyone wants kids to have easy access to this stuff, but that any of these laws will either be (a) unenforceable and useless, or (b) draconian and privacy-destroying.

Surely not.

Imagine: government sells proof of age cards. They contain your name, and a unique identifier.

Each time you sign up to a service, you enter your name and ID. The service can verify with the government that your age is what it needs to be for the service. There are laws that state that you can't store that ID or use it for any other purpose.

Doesn't seem impossible.