If you're an adult that's responsible enough to be a parent, you're responsible enough to carry ID.
Its nothing to do with responsibility. The law does not require me to have an ID card in order to purchase alcohol. I do not drive. Plenty of people do not or cannot do so, so do not have drivers licenses. Neither am I happy to walk around with my passport on a casual basis in central London (or anywhere unless strictly necessary ie., going to the airport). Our freedoms of privacy are pretty much vanishing. I do not want to carry an ID card unless the GOVERNMENT decides (preferably via a referendum) that it is mandatory for everybody to do so. Then, in the utmost reluctance, I will. I am 40. Not 30, not 25 and definately no where under it. I do expect shop staff to use their common sense, and discretion. I assume these are qualities they must possess, hence why they were recruited in the first place, unless stores don't screen for these attributes anymore.
It's got nothing to do with sympathy for staff. I have every sympathy, and think its rediculous to pass the buck to them, but if someone is just demanding ID because they've switched off, and can't be bothered to look at me properly, I will challenge them the way they are challenging me. Politely, but firmly.
I too work a shitty shop stewarding at events in all weathers, for hours on my feet for minimum wage, but I don't treat mature people like liars, or children. I use my common sense. In fact, it's precisely because I work with the public that this basic inability to apply common sense annoys me so greatly! Especially if I am dropping on my feet tired, and just stopping to pick up a couple of items and maybe one cider for the evening and someone decides to choose that time to be overly zealous about ID, despite me clearly looking no where near 25, let alone under it. And even when they've realised they've made a mistake refusing to u-turn, making my shop stressful entirely unnecessarily.