It sounds simple when you’re not a mathematician or cryptographer, but the academic study of these systems reveals that it’s probably intractable:

Privacy-Preserving Age Verification and Its Limitations - Steven M. Bellovin

Cory Doctorow wrote an article shortly after this paper released that explains the situation quite well: https://pluralistic.net/2025/08/14/bellovin/#wont-someone-think-of-the-cryptographers