Before uni I worked in Sainsburys for a year on checkouts, the ID thing was the most anxiety inducing part of the job. The training was terrifying, we were given examples of staff who failed to ID when it was a test and were fired on the spot, arrested, given a fine and would never be able to travel abroad again.
I was 19 at the time and once ID'd a woman who turned out to be 35. She was shopping with her mum and they both just laughed at me. I remember working on the self checkouts once and didn't ID a guy who looked old enough from behind but when I looked properly (after approving it) I realised he looked much younger, but he had already left. I only worked weekends, so I spent the whole of the next week in a state of constant anxiety, having visions of walking in for my shift next weekend and being marched straight upstairs to a manager. I'm an anxious person, but that part of the job nearly gave me a nervous breakdown!
I would say ID'ing someone who's 40+ is ridiculous, but they really do terrify the staff to the point where its easier just to ID everyone. The fear of ID'ing someone who's clearly old enough and having them be angry with you is also a source of anxiety, and the 'do I or don't I' debate that goes on in your head as someone youngish looking (but then again not super young so could be 30 or even 40) approaches you with age restricted products is awful...
Now whenever I'm in a supermarket and it looks like the cashier is eyeing me up and down uncertainly, clearly not knowing what to do, I just get my ID out and give it to them, and I try to be understanding.
Having said all that, stories of 50 year olds being ID'd, and that story about a woman being refused a bottle of wine because she was shopping with her baby are riridiculous. I can only think that they've got even stricter since I worked in that role and staff are thinking they're better off just asking everyone for ID as a blanket rule.