Yes I agree with Mikey. You should have left the beer (along with all your other purchases) at the checkout and walked out.
"...and what happens to staff members who don't ask legally required questions?"
They are not "legally required questions", jno. An extract from the government's guidance on the "age verification policy" requirement which is associated with liquor licensing:
"As a minimum, the premises must have a policy that requires people who appear to be under the age of 18 to be asked, before being served alcohol, to produce identification..."
Note the important phrase "appear to be under the age of 18". The requirement is to ensure that they do not serve alcohol to anybody under age, not that they must seek verification of the age of everybody who wants to buy alcohol.
This is yet another example of employers "gold plating" laws and regulations and training their staff that these gold plated requirements form part of the law. These misconceptions spread to the general public (as they seem to have here) and so the nonsense endures.
I related a tale a few days ago of a high street travel agent asking me for proof of identity before they would exchange a £20 note into euros. I was told "it was the law" when I knew quite certainly that it was not. More ridiculous nonsense perpetrated by stupid employers too lazy to train their staff properly.