Idea #715: Use age-discriminatory noise devices, which don't work on people-in-their-20s, to find out if people are permitted to buy alcohol or not.
This would reduce the need for:
a) Patronising notices/supervisors saying that I'm "lucky enough to look young" - too young for booze.
b) Ever-increasing age levels for ID. Look under 18? ID please. Look under 21 and so maybe under 18? ID please. Look under 25, and therefore under 21, and therefore under 18? ID please. Look dead, and therefore maybe alive, and therefore under 40, and therefore under 18? ID please.
c) Babies in supermarkets.
Hell, you could probably automate this somehow, and have drink-dispensing vending machines on the streets.