For simple yes/no questions ("Is over 18?", "Is US resident?") then you should look back to David Chaum's blind signatures and the work that came out of that back in the 90s. The math is super-simple to understand and there are a ton of even easier metaphors with envelopes and carbon paper that you can use to explain to your grandmother. Once you get someone to grok blind signatures it is easy to lead them to zero-knowledge proofs.