I'm 54. A UI with clutter, or carousels that move too fast, or text in a small font (or one with low contrast against the background color/image) is hard to use. I also have middle-aged memory issues, which only get worse with age: I'm far less able to hold multiple bits of information in working memory, so I can get demotivated and give up when a website or app expects me to remember seemingly irrelevant details (hint: disabling copy/paste in password entry fields is a great way to lose me as a customer because I rely on 1password and know enough to use strong passwords — but I can't memorize them).
Apps and websites designed by twenty somethings tend to assume twenty-something levels of cognitive performance and sensory ability and reflexes which older users once had but have now lost.
And so on.
… And before you comment to say "well, don't use the app/website, then", bear in mind that it's increasingly impossible to avoid filing your tax return online, dealing with your bank and credit cards via an app, ordering supermarket deliveries online (especially if, as with many elderly people, you're mobility-impaired) …
There's a point at which bad UX design becomes life-threatening. This is well understood in human factors engineering in e.g. the aerospace sector, where aircraft cockpit controls are carefully designed to be where and do what pilots expect. But there comes a point at which badly-designed smartphone apps are going to become an actual hazard to users who are unable to use them as their (younger, able-bodied) designers expected.