If there's a simple piece of software that can be installed, it's not meaningfully increasing the barrier. Also, there are negative consequences to introducing "rules that you're expected to break" like this. It makes the law unserious.
And if it's a phone app, it's not going to be on app stores and you already know the person giving you the app is a criminal.
So you're installing an untrustworthy app to risk criminal charges, and the customers of this scheme are kids who mostly don't have a lot of money.
So the schemes inherently add some traceability, which makes the tokens no longer actually anonymous.
This is the back door used to make the tokens double as ID tokens.
1) You give a teenager your full credentials. Teenager is careless, as teenagers often are, and posts something revealing who he is. Cops have option to search teenager's phone, see who you are, and at least revoke the credentials.
2) You install a relay app on your phone, for money. Now you've installed an untrustworthy app from a criminal, who might hack you, or might be arrested and reveal details of your device and where they're sending your money.
Neither scenario happens because the age verification is traceable.
3) Your credentials get stolen, and used in a foreign country to implement a relay scheme.
This one, I admit, my scheme can't do anything about. But this means our teenager has to pay a foreign entity. Teenagers can also pay foreign porn sites directly, if porn is our concern.
On top of that, the age verification systems we've seen so far have their own security holes that teenagers are exploiting without having to pay anything.
My personal view is that the whole thing is ridiculous and we shouldn't bother with any of this. My point is just that we can implement reasonably good age verification without eliminating anonymity on the internet.
Just offer the user some money if he installs some "trusted" app for age verification token sharing.