I've been in a transitional state career-wise, partially by choice, but I think there's two major factors not really mentioned by the article, and in fact, factors I think the article reinforces.
The first is ageism, like you say. But I think ageism is a lot more subtle than other things, because it's not only an assumption of age-related decline, but more generally an assumption that people can't change. So if you become known for X, it's hard for people to see you as anything but that. You get typecast, and those assumptions become societal constraints. It's like there's career "rails" and if you go off the rails, people don't know how to process it.
The second is a sort of lack of recognition of how much randomness and fads plays a role in certain fields. What I mean by that is that the reasons some people attain relative notoriety and status is often due to butterfly effects or societal viccitudes, and these become revealed only over time.
I think in general, there's a very superficial way we process people and their potential for change or new work. Even when they're capable of it, they're often prevented from doing so because of assumptions and stereotypes.
Sure, cognitive decline happens, but it's not as dramatic as it seems in this article.
Sometimes I feel as I've gotten older it's just a process of seeing more of the wizard behind the curtain. I read articles like this and get depressed only because it perpetuates the societal ills that lead to the problems in the first place.