Not to derail too much, but...
In the U.S., one qualifies for Medicare at age 65. There’s a “Medicare Wellness Test” that doctors are encouraged to give their patients, though it seems not all do. It includes a cognitive test like Trump bragged about, where you draw a clock, have your memory tested, object recognition, etc.
It also includes other questions, asking about abuse and, relevant here, falls in the last year. At age 71 I’m at zero so far this year. But often there are one or two. Stepping off a ladder awkwardly. Tripping over a dog leash. Slipping on snow or ice. Missing a curb. That sort of thing. And, yes, stumbling on steps.
There’s no doubt there’s a general decline in balance, coordination, reflexes and strength with age. It probably starts much earlier, but for me it became undeniable after age 60 or so. I still fly, ride motorcycles and bicycles and am generally active, but I have to be more careful doing what used to be simpler tasks.
All of which is to say, Joe Biden stumbling on the steps is hardly unusual for a man his age. It happens.