This is a law-respecting tavern.

Take it even farther, not all species get hammered on the same chemicals. “Barkeep, a glass of your finest room-temperature-aged milk, please!”

A rule book for all the different 'coming of age' ceremonies and that the proof of those are.

"Dar'grock the destroyer, soooooo you should have... A....oh! The skull of a foe slain in 1 on 1 combat"

Why you need see Dar'grock's Foeskull? Dar'grock forget Foeskull today, but look at Dar'grock face, he have full beard so Dar'grock clearly come of age already....

Please, Dar'grock will bring Foeskull next time!

I took a sacred oath, sir! I can't serve you Mr 40 year old Drow or I become an oathbreaker. Sorry, not sorry.

Meanwhile, "Let's see... anthropomorphic mosquito... Let me see that ID. You're 2 hours old? That checks out, here's your bourbon.
- Tbh when your life is that short, you can't really see the consequences of your actions, so you might as well get wasted 24/7
- It takes five days for them to go from a larva to a pupa. They're definitely not an adult, and they couldn't be confused for one.
I know a lot of people homebrew this to be different in their games and that's fine, but just to clarify to anyone curious, if we're talking D&D 5e, elves mature at the same rate as humans, and the reason they only consider themselves adults at 100 is purely cultural.
Nah man I'm part human on my fathers side so it's fine!
You broke your oath with the last sentence.
- How does sarcasm break my oath? As a lawful paladin I don't care if they're mad about being blocked from doing something illegal.

so it says here cyborg elvish catboomer
hmm...

I can only offer a dragon's vegan tonic with 2 Karmotrine or a pearl of shapeshifting water

if you aren't satisfied with our current selection please come back in 2 cycles, I'll make sure my successor will be able to better serve you with an expanded menu

Ah yes "optional" karmotrine. Which to me means fill the rest of the drink with it.
- lol😂
  
  it's time to mix drinks and change lives

I feel like it would be the same as countries outside the US: If you can reach over the bar to order, you're allowed to drink.

So it must really suck to be a gnome or halfling.

Ah, yes, "outside the US", where there are no laws...

https://vinepair.com/articles/legal-drinking-age-world-map/
- Funnily, despite what OP suggests, it's if anything MORE difficult to get alcohol outside the states, especially for teens.
- Everywhere I've personally been outside the US either didn't have or at least didn't enforce any age for purchasing alcohol.
  
  For reference, those countries were Ireland, Germany, Mexico, France and Canada.
I live in the UK and you still get ID checks for many places. It's not as lax as you make out, although it definitely depends on the specific establishment you are talking about.

Mayfly enters bar.

Bartender asks for ID.

Mayfly: Do I look like I was born yesterday?

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

That’s not even what happens here though. Just because you can drink in Canada at 18 doesn’t mean I can go to the States, you’ve gotta be 21.

Not sure if Canadians and Americans are different species though.
- We are.
- Me neither, but we mustn't rule it out yet.
Of course, the values vary depending on which pub guild is controlling this particular pub.
Different species may have different ages of brain maturity at which point alcohol won't pose as much of a risk of stunting their mental development. Elves for example settle into adulthood at around 100 years of age.

So a responsible legislature would codify in its laws what the minimum drinking age is for each species based on science's best understanding of their physiology.
- Dragons don’t really ever stop growing, although if there’s one in your bar you have much bigger problems.
Not really a good analogy, because the age at which you can drink is based on where you are, not in where you were born etc. An American 18 year old could go to Canada or well pretty much anywhere else in the world and they can get as drunk as they like.
- age is based on where you are, not in where you were born
  
  Age is based on when you were born, not where you were born, unless if you were born orbiting black hole /j
Hell, just different parts of Canada have different drinking ages
- There's a reason why university students in Ontario like to go to Montreal to party, after all.

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.

"...Unless you're dextro like me. Then it'll kill ya."

Not fantasy, but in the SciFi book the Draco Tavern is about the only alien bar on earth wherein the owner has to be careful not to feed his guests any poison that would be delicacies on Earth.

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.

A universal biometric verifier checking against approved ID cards could be interesting in a setting like that

What are like characters ? characters that are similar ? alike ?

30 and uhh... half elf. Not full elf. Totally half.