At least the EU is somewhat privacy friendly here (excluding the Google tie in) compared to whatever data sharing and privacy mess the UK has obligated people to do with sharing ID pictures or selfies.

Proving you are 18+ through zero knowledge proof (i.e. other party gets no more information than being 18+) where the proof is generated on your own device locally based on a government signed date of birth (government only issues an ID, doesn't see what you do exactly) is probably the least privacy intrusive way to do this, barring not checking anything at all.