Only time I've every been asked for proof of my age was just recently (I'm 57, btw) when buying a trolley load of alcohol (and other party fodder, like finger food) in Tesco Metro. The check-out assistant was reak cute too, and wanted to ask why such a purchase, so she used the proof of age gambit as a conversation opener. She said 'Why buy this here, and not at the larger store in Xxxxxx, it would be cheaper' I said the party was in a local rented apartment that very evening. She said 'I finish at 8pm, can I come'. She had such a cheeky smile - so I said said 'Yes, probably - I'll call you just after 8pm and tell you more' I did. Although it was a 'working' (private dance) party she was perfectly OK with that, and came!
She stayed with me (part-organiser) after the party ended - and we left the apartment at 11am. Yes - she went straight back to her next shift at the supermarket!
:-)