The offline version proposed doesn’t even work. If the government issues untraceable “I’m an adult” cards there’s nothing stopping someone from acquiring one and immediately selling it to a minor. There’s also realistically nothing stopping someone from acquiring multiple and selling them to minors. “Oops, I lost my adult card again. I need a new one.” The solution is of course to make them revocable which means traceable.
The same applies online/digitally except that you can very likely distribute the same ID to many minors without getting a new one allocated.
I’m in the “we should protect kids online” camp, but I am not sure there’s a real way to do it without compromising privacy for everyone.
Untraceable is your invention and obviously absurd and unworkable.
No scheme is going to issue cards/tokens that can be traded between people, that's obviously unworkable.
Untraceable is for the consumers of the cards obviously, but in order to have trust they need to be traceable for somebody to verify them. The government can already determine who you are and already requires you to prove that at certain points, this is not a new invention of government it is required for governments to function.
Protecting kids online requires attention from their parents, not from the government.
If the kid knows how to buy a certificate from a shady website, then probably he his old enough to watch porn and other disturbing content. That does not mesn the system should be thrown away as a whole.