they could, but a law enforcement agent looking for a suspect will send a lot of subpoenas to every porn site. When a porn site says "we wipe that data instead of storing it," the law enforcement agent will say "what do you mean you wipe KYC and identity verification trails once you get them? Are you letting sanctioned people use your site and covering your tracks?"

Similar thing happened to Valve; people were trading gun skins, and regulators fined them for not having AML/KYC controls because the state argued "the business didn't do enough to stop money laundering."

This trickles out to porn companies (and the vendors that use them for identity verification), and implies that they need to store this data to prove that they didn't delete it to help terrorists.

Does this require users trust several parties? Any one of which could sell out, get a court order, be tapped en mass by the NSA?

It would be helpful to be able to digitally verify different types of identity. Where I live, how old I am, my real name, my nationality, etc. Give the user control over what information is being verified.

THe simplest way is as follows:

1. There's a provider that already has your data (it could be the government, a bank, a phone carrier etc). If more than one provider is supported, there's a list of trusted providers somewhere.

2. Whenever a website needs an age check, it asks you to authenticate with one of the trusted providers. The provider gets a challenge (a random string).

3. If you authenticate successfully, the provider uses their public key to provide a cryptographic signature of the challenge. This signed challenge is then transmitted back to the website.

In a more advanced version of this system, the website also provides a boolean expression, like `country_of_residence not in forbidden_countries && (age > 21 || (age > 18 && country_of_residence != "us"))`, and providers promise not to return successful responses for users who don't fulfill the expression criteria.