I think we just have to accept that no solution is 100%. You just need some balance between success and side effects.

As long as people can communicate, it will to some extent spread. So really, the only truly 100% solution would be a couple of nuclear bombs all around the world. Look at that, crime rates went to 0!

I am not really intelligent or knowledgeable enough to provide best-balance solutions.

Anyway, as far as limiting access to porn, perhaps it could be done privately. You could have some trusted verification platform, perhaps even government provided, deal with it with cryptographic signature, maybe as I said, I am too dumb for solutions.
My idea is, the website that wants to verify your age knows the public key of the government provided service used for this purpose (who already has your data anyway), and generates some random data in base64. You copy that, paste it into the verification website, they sign it, and give you the base64-encoded response. You then paste the response into website that wants to verify your age, it verifies the signature and then that you're 18+.

Basically just trusted authority saying "verified being 18+". The website doesn't know who you are, verification system doesn't know what you wanted that verification for.

Of course, you could verify that for anyone else, but I can also walk into a shop, buy cigarettes, alcohol and give them to a kid. Not 100%, but good enough.