This is not an accurate summary of what the study actually found. The article proposes a high-level explanation for how one aspect of learning changes with age.
The actual cause of this high-level change could be lower-level age-induced differences in neurotransmitter levels or in neuronal function and connectivity. As the article's summary points out, the actual biological basis needs to be investigated in further research.
It's also important to note that this study found a plausible explanation for changes in learning performance under specific circumstances. This is a very important qualification.
Learning in general is believed to involve many different interrelated mechanisms. It's well established that many different cognitive abilities (perhaps most importantly working memory) diminish with age, and some of this degradation is likely related to natural cerebral shrinkage (a healthy 75-year-old has a 15% smaller brain than a 25-year-old).
I interpreted the paper slightly differently: The older you get, the harder it is to tell whether you understand something. The authors are not sure exactly why:
> Why do older adults fail to represent sufficient levels of uncertainty? One possibility is that representing appropriate levels of uncertainty requires a cognitive and/or biological resource that decays across healthy aging. One obvious candidate for such a resource is working memory capacity. ...
> Another possibility is that older adults fail to represent sufficient levels of uncertainty because they have an aversion to uncertainty or the mental effort required to represent it. ...
> The crucial factor limiting uncertainty representations in older adults could, and at some level must be, biological in nature. One candidate for such a limiting factor is norepinephrine ...
> Although it is tempting to link age-related changes in representing uncertainty to reduction of a single neurotransmitter, several alternative biological accounts exist. ...
The truth is, most people seem firmly in the grip of illusory competence at most times, and it's not a shock that as life progresses that becomes crippling. I'd be shocked if people somehow magically changed in old age, rather than just distilling who they've always been.
Why would you be shocked? It's a medical fact that a lot changes biologically as we age, and it's also a medical fact that biology can greatly affect the mind. There's nothing magical about senescence.
That is not really clear to me. Biological changes can give us depression, irritability, mania, sluggishness, dementia and many other less-than-desirable mental states. What kind of change were you referring to that is fundamentally different from the things we know biology can do to a person?
Older people are more often fulfilling the above premises, and thereby the above conclusion.
This means: Worse learning performance is not a consequence of age per se.