usually the job of the door staff, then management, the bar tender can refuse or ask for ID, but predominantly itīs the door staff or managements position to keep an eye on age
My partner who was a door man got 2 people wrong, although, would have asked ID on one & yet was still over 18, as the age is 18 in the UK...
This actually making it easier to tell usually, although the 15yr old girl was a bit like omg, thought maybe 19
