Comment by Hizonner

You have to trust someone to verify age.

You don't have to trust somebody not to track how the resulting credential is used. And that is what "zero knowledge" means. It means that after you finish the protocol, nobody has learned anything but what they were supposed to learn (in this case, "the person at the other end of this connection is over 18"). If it leaks anything else about the person, it's not zero knowledge. If somebody learns which of the issued credentials was used, it's not zero knowledge. If parties can collude to get information they're not supposed to get, it's not zero knowledge.

It's a technical term of art, not some politician's bullshit. And it isn't complicated to understand.