That is not true and "true zero knowledge ID check" + "age verification" with blind signatures is what's being implemented by the EU ID project.
So someone's id leaks. It happens. In EUDI there are things called "cryptographic accumulators of non-revocation proofs". If your ID leaks it goes into the accumulator. Similar to the certificate revocation lists. During check, you include claims "im over 18" and "my id is not in the accumulator".
This is included in the standard.
This is also (I can only assume) one of the reasons why EUDI wallets require play integrity / attestation / secure element on the device. So your private key won't be easily leaked and no one can steal your ID.
What happens when someone sets up a marketplace where people can sell those blind signatures using their ID for $2 each? And then kids just pay $2 to have someone else blindly use their ID to validate the account, because supposedly the system is structured so that nobody can tell which ID was used or tie it back to the account?
In theory you cannot export your private key from the device (from the secure element), so for each $2 someone would have to quickly unlock their phone, scan code via the app and so on.
So like CT logs, but several orders of magnitude bigger? I thought centralized TLS revocation lists failed due to scale. How will this differ?
That's unnecessarily reductive.
Yes, every solution will have problems, but not all solutions have similar problems.
If a solution has problems such that it can be immediately reduced to security theater and bypassed by any teenager who cares, it's just extra hassle and privacy degradation for the rest of us.
These details matter. If a weak solution is regulated into law and the government discovers kids are easily bypassing it, they will immediately pivot into requiring more restrictions on it.
We've had decades of age gating being "are you 18+ or not" yet it is only now that talks of something more enforceable are coming up. This discussion is largely about how one can create a sense of safety and protection. For the more extreme end it's face scans and submitting ID. Even though these are bypassed by any teenager who cares they are still being pushed seriously because it instills that sense of safety and protection for children. Security theater is just a part of managing the internet and not going away unfortunately.