The logical conclusion to Morrison's response is that had he confirmed with a simple "Yes I am over 21" she would have let him buy his wine ~ thus anyone else who replied to the question "Yes I am" would also be served alcohol .... no matter what their age.
It is not the asking of the question, it is the manner in which it is phrased. I frequently get asked by telephone callers (banks etc) to confirm my telephone number and/or date of birth "For security". I always say "Yes I will ~ you tell me what you believe it to be and I will confirm or deny it. If, on the other hand, you want me to tell you what my DoB is then bloody well ask me and then YOU CAN CONFIRM IT AGAINST YOUR RECORDS". .... Fvcking cretins in management positions are to blame because they write the scripts and insist on their own piss poor education taking precedence. I will not stand for it and hang up the phone or walk away from the cashiers.