It does sounds like that the author is not aware of existence of zero knowledge range proofs for identity based authetification (anonymous credentials). In essence they work as follows: a third party vouches a commitment for your birth year into a commitment with a signature. This then can be used for to be shown to a service provider along with a range proof to ensure that the age limit gets respected. That way service provider does not learn your year while can ensure that all members respect some required age limit.