I think, at least from my interpretation of it from being in China and having Chinese family, that something like underage drinking is seen more as a family issue, than a legal issue. What stops the 16 year old from drinking? The fact that their friends / family will see them being drunk, and think less of the person and their family. A 16 year old being drunk in public is family issue. Sure, the cops will intervene at some point, but China has very little drunken / raucous public behavior than the west does.
they were big on the opium, as I recall
That's what happens when you get systematically manipulated by the biggest empire in the world smuggling in illegal drugs...
What if I want to buy copious amounts for a party? Or there was a discount so you want to stock up? This seems a bit shortsighted, it is not always the case that if you buy something then you need to consume it right away.
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.