I think your parenthetic comment hints at the truth. There isn't a typical age, rather a wide age band where individuals may experience different declines.

With dementia, the descent can seem very rapid. One year, they are functioning well and adapting, but suddenly you realize they are in deep trouble. For a while, their adaptive energy may be devoted to covering up deficiencies. So, when this coping mechanism fails, the struggle suddenly emerges for all to see.

But, some people may fit the description of "unable to learn new things" long before that sort of dementia is relevant. They may be depressed, or may have a prideful personality which avoids the unknown because any trial and error learning is embarrassing. Frustratingly, it can also be hard to get elders in this category to go anywhere near a doctor for a better evaluation. And without a better understanding of what is going on, it can be difficult to identify an appropriate intervention or care approach.

While it is true old people slow down, I think interest in learning is the bigger factor. I know old people who enjoy new toys and get them all the time, even though they struggle [with something younger people find easy) they enjoy the learning and figure it out. I know old people who have no interest in learning anything new in general - yet when presented with something new related to their hobbies can figure it out quickly).