At first I laughed, but in honesty I can’t think of many better verification methods.
My concern with it is the cost and requirement for non-drinkers to acquire alcohol. It’s pretty insensitive to ask a recovering alcoholic or a Muslim for example to go into a pub and buy a beer. Maybe have it as one possible verification method?
You could also get a a piece of paper notarised by a JP.
‘The individual known on Aussie.zone as Gorgritch_Umie_Killa has presented identification to me that demonstrates to my satisfaction that they are over 18’ (signed and stamped).
But neither of these methods are technical solutions.