Disclaimer 1: OP I absolutely hate everyday ageism. I prefer not to discuss my age, especially in certain situations e.g. at my karate classes. I won't have my ability pre-judged by how old I am.
Disclaimer 2: I know the purpose of the supermarket DIY checkouts is to reduce staff.
Disclaimer 3: I know many people can't afford the latest technology (although many people of all ages claim to be badly off but they have all the latest tech!)
In our local Sainsbury's, I use the Smart app on my phone. You scan as you go and pay at the Smart checkout. They must assume I'm smart enough to manage because they never offer help. They will come and help if the red light is flashing e.g. for age verification on alcohol, paracetamol etc. But, if things go wrong for some reason, (a rare occurrence) you have to wave and jump up and down until they spot you! Occasionally, the till informs you that your shopping must be verified, to catch possible shoplifters I suppose. The assistant always reassures you that it's a totally random thing.
I told my hairdresser, who is a very few years younger than me, that I use that option to save money. Sainsbury's offers targeted lower prices, on things you buy regularly, if you use nectar and the Smart app. I save, on average, about £10 per month. She told me off! She said many older people can't manage the DIY options and I shouldn't get my shopping for less than people who need to use the operator checkouts! As far as I'm concerned, that money is better in my bank account than in Sainsbury's. I'm on a pension!
Older people often say they can't manage technology like smart phones and DIY checkouts. In many cases I think it's fear of change. Change isn't always bad! Embrace it (if you can afford it)!