So we are ignoring jurisdiction? … Well… even if we ignore the possibility of an infinite universe, tracking hundreds of thousands of legal systems and species biology (what is intoxicating to whom, the cognitive self-development thresholds, how much of a substance one may serve to whom) and being able authenticate/verify an ID (we have photo-ids, but maybe others are different)… like a slime-species that identifies each other by smell… or taste… I’m confident that for any interesting set size, this would actually be an unsolvable problem.

Mayfly enters bar.

Bartender asks for ID.

Mayfly: Do I look like I was born yesterday?

This concept goes so well in a spelljammer like setting. Imagine the Laughing Beholder with large Luigi doing this. “Okay, what sphere are you from again? Okay, next I need your planet, then your species, then your age.”

EDIT:

Noncombat or low combat adventure idea that just came to me based on this. A group of grifters has conned Large Luigi or your version of this bar owner by posing as an interplanetary or interplanes alcohol enforcement agency. They pulled a fake sting and made off with something precious of the innkeepers as a fine. The party either hears this from the barkeep when they order or from the other patrons who are fed-up with the arcane ID system and long waits on drinks. Only recovering the fine or capturing the grifters can the situation be set right.

Legal drinking ages are such a modern invention though. It’s kind of funny but a weird fit for sure.

“…Unless you’re dextro like me. Then it’ll kill ya.”