No ID was required, the person on the till just required a verbal confirmation that the man was over 21. It may seem a pointless question in this case, and indeed it is, but this is hardly a unique case. If you refused to answer equally pointless questions put to you by customs or immigration officers you would be very quickly arrested.
Actually, it's posted elsewhere that they were specifically looking for a confirmation of his age, in the form of id that proved it. It wasn't just a verbal request.
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