[Grimeton]

They could just issue a x509 cert with an above 18 attribute and nothing else.

You're not 18 yet?

No Problem we just give you two certs with different valid from/to ranges that overlap and don't give away your birthday.

Problem solved.

[Borealid]

Steelmanning the opposing position: one adult shares their certificate with every child on the planet.

Because it has no attributes (not even a unique serial number that could be used to track it) the whole scheme is now defeated.

[fragmede]

Which, adult is merely someone who's just turned 18. What're the chances that any of them just post the thing up on 4chan? (I'm going to go with 100% chance of that happening.)