As an aside a young female colleague of mine (over 18 & possibly over 21 too at the time) was refused alcohol at a branch of Tesco when I happened to be at the next till. I offered to assist by verifying that she was over 18 and producing my Police warrant card & other bona fides. Was this acceptable? - No, it was not! Also I was informed that I personally was being refused the sale of any alcohol (I did not wish to purchase any so the point was moot) in case I later supplied my colleague with it. It was all rather surreal.
This in one form of behavior I feel is truly vile, They have no evidence that you would supply the aforementioned product to the your colleague, sales are made to an individual, not a group, whom that individual associates with, no matter their age, is not evidence that they will or wish to supply alcohol to them. The law simply catches out the innocent, whilst allowing those who wish to circumvent the system can get away with it.
Interestingly most staff have a different reaction when a mother and child go throug a till with a bottle of wine... Pure age discrimination.