I should not have been surprised at this. The attitude to alcohol is quite different here in the US from the UK. It is illegal to drink in public - which has led to the time-honored tradition of swigging from a brown paper bag in order to outfox the police. Many waiters and store assistants don't consider it worth their jobs to judge on appearance, and will ID a granny as much as a tetchy Englishman in his thirties.
So I have learned my lesson now, and carry ID with me wherever I go, positively wanting to be asked to produce it. If I am, I do so with a flourish, like a schoolboy handing in his homework on time. If not, I conduct the rest of the transaction in a frosty silence, cursing my latest hairdresser for not adequately addressing my receding hairline.
I have not been back to the store from that evening. But if I get to the point that everywhere else has stopped asking for my ID, I may just pay them another visit.