Even if you
could do this in every single country (it would already be extremely hard to actually do this in the United States reliably, and I can only imagine it is basically a non-starter in a lot of developing countries) it does pose so, so, so many problems.
- How can you ensure the system can't be abused if there's no identifying information passed? Don't get me wrong, this is also a problem with current systems, maybe even worse. But if it's privacy preserving, ... Almost all kids under 18 have parents or guardians. Almost all of those parents or guardians are 18 or older. So literally all you have to do to bypass age verification is steal their ID for a few minutes? There are also a myriad of solvable problems that aren't guaranteed to be solved without care, like ensuring that the same ID is not used 100,000 times.
- This is a job that is best suited for the government to handle. The internet is global though, and there are a lot of governments. In the U.S., there is in fact not one federal ID, but instead we use state IDs. I assume that means you now need to handle around 50 different state IDs to be able to verify someone's identity, but it actually gets even worse than that, because some people will have IDs, and some will have drivers licenses, because oddly enough that's just how we structure IDs here. People without drivers licenses may have state IDs which are often intentionally visibly distinct to make sure they can't be mistaken for the other. In states I'm aware of, you'll never have both, the driver's license acts as a state ID if you have one. Now scale that to every country on Earth.
- As insane as it may sound, there are plenty of people who don't have essentially any form of ID. You might think I'm over-estimating the numbers with "plenty", but even just in the United States, it's literally over 2.5 million, off the top of my head. (No idea what the best source is here.) The closest thing we have that every citizen is supposed to have is Social Security, but that isn't really usable as a form of ID for various reasons. (And frankly it's a pretty terrible means to verify someone's identity at all anymore in the Internet age, but oh well.)
I'm totally sympathetic to the fact that people really don't want their kids browsing porn on the Internet, but children basically can't pay for Internet access or afford iPhones. I think it's insane that people keep suggesting using advanced cryptography, zero-knowledge proofs, privacy pass tokens or whatever else for a problem that so clearly needs to be solved socially and not technically. (And obviously, only the surface-level aspects of this are really about porn. We all know it's deeper than that, and if it wasn't, the UK would readily exempt Wikimedia from these requirements. I hope nobody here is deluding themselves into thinking this is a noble effort.) You are literally giving your children a device that can easily obtain porn and letting them use it unsupervised. It's not like it was a secret: Avenue Q told you everything you needed to know. I get that raising kids is hard and society pressures you to do this, but isn't that the problem you'd rather tackle?
The problem is that we've let this idea that you can solve the problem like this enter the mainstream, and now that we have, even smart and reasonable people may accidentally convince themselves that it is tractable just because it is technically feasible to devise such a system. This is bad because we're going to waste a lot of energy repeating ourselves on thinking about the entirely wrong way to look at things.