A friend and I came to discuss about the formal legal age for alcohol at public bars and clubs in Hathian, since some people seem to say it is 21 to buy alcohol in stores, but 18 at bars.

Campus is one thing:
"No drinking on campus if under 21.  Venues that serve alcohol should ask for appropriate ID." (2023 CU Campus Guide)

But what about Lou's, Twister, etc and please remember I am asking about the formal age so no need to refer to that no one is getting carded anyway or about the actual age of serving staff and similar things that have been discussed before.

Someone (second hand information, unfortunately) said Hathian follows the laws of RL New Orleans in this matter. If so, this is what I found:

"New Orleans is a 24 hour city with an open container law. The drinking age is 21. The police are tolerant of drinking in public in the French Quarter as long as you do not have an open can or glass container. Bars and restaurants will give you a plastic "go cup" when you leave their establishment." (New Orleans Police Department online NOPDOnline.com)