Zero knowledge in such a system requires a minimum of 3 independent parties. There are quite a few solutions out there, I think the most developed ones are online voting systems, because tracking and de duplication is essential.

The impossibly high bar they set "Perfect" at in order to make it the enemy of good, and fight against any progress being made to keep children out of adult spaces.

That being said, it's my personal opinion that I'd love to simply have my device store a token and send it to any site when requested. I'd then like those sites to give me toggles to remove all non-verified content - and therefore my internet experience could be sans-juvenile squeakers.