The first time I visited China I was under 21 but I had heard the drinking age was 18 so I went to a convenience store to buy a beer. Person running the till was probably 12 and didn’t say a word or ask for ID. Unbelievably lax compared to the US sometimes.
I generally think it’s easier and more effective to track the outputs rather than the inputs: you don’t need to track how many beers they buy, just outlaw public intoxication. And enforce that law.