Those things come with a big convenience and implementation trade-off that slows adoption.

If it's hard to export for technical reasons (eg. Needs to be in a tpm) then that adds hardware requirements and complexity and makes it difficult to log in on other devices. If it's a software thing, then it's rippable. Either way "install our government app to watch porn" is not an enticing prospect for people.

Aggressive rate limiting is also frustrating if you want to log into multiple things and it keeps blocking you because you're using your key too fast, but if it's not aggressive then it likely won't be effective unless all the kids sharing a key are trying to use it at once.

If it's a temporary thing where you have to auth with the government to get a fresh signing key that expires, you have the issue of having to sign into the government when you want 18+ content which is super uncomfortable.

I can see it being a browser-based thing set up a bit like video DRM but that would still need to talk to a government server each time for a temp key (like how licence servers work) and you'd need to be logged into their systems. It might still be the best option but it does still leak "X person wants to access 18+ content right now" to the government.

I'm really interested in seeing a technical/cryptographic solution that actually works but so far I haven't really and I'm starting to doubt that it's possible.