To date, proving you are old enough is almost always (over-)implemented by having to reveal your legal identity and the exact date you were born.
If the whole world goes down the route of AV / age-bans then I hope we at least get some kind of escrow service where you visit an official office, prove your age to a disinterested public official, and then pick a random proof-of-age token out of a big bucket. The bucket’s randomness is itself generated when it was filled up with tokens at the Department of Tokens, and maintained by a chain of custody.
You could do it on polling day: ballot boxes get sent out to polling stations filled with tokens and get sent back filled with ballot papers, with the whole process watched by election monitors. Now everyone has (a) voted (b) picked up a proof of age/citizenship token. It would improve turnout, though I believe that’s already mandatory in Australia.