Researchers here propose that, regardless of other forms of physiological damage that degrade the brain with age, we might consider that simply having a longer period of learning behind you could cause a decline in cognitive ability. This is plausible in the theoretical sense; there are plenty of other systems in the body, such as the immune system, that experience declining effectiveness over time not only because of damage or wear and tear but also due to the way in which they are structured. Evolution doesn't produce perpetual operation, but rather systems that have peak performance for just long enough to get by - and that can result in a system that eventually grinds itself into self-destruction simply through continued normal operation.
Theoretically plausible and a meaningful effect are two very different things, however. The weight of evidence linking forms of physiological damage in the brain with levels of cognitive decline strongly suggest that damage is the overwhelming cause. These researchers are arguing from the point of view of models of how the brain works, rather than any more robust data. So I'm not giving this line of thinking too much serious consideration at this point in time. But we shall see:
In what follows, we consider the question of whether one might reasonably expect that performance on any measure of cognitive performance could or should be expected to be age- or, more specifically, experience-invariant. We shall suggest that, since the answer to this question is no, many of the assumptions scientists currently make about "cognitive decline" are seriously flawed and, for the most part, formally invalid.
We will show that the patterns of response change that are typically taken as evidence for (and measures of) cognitive decline arise out of basic principles of learning and emerge naturally in learning models as they acquire more knowledge. These models, which are supported by a wealth of psychological and neuroscientific evidence, also correctly identify greater variation in the cognitive performance of older adults, and successfully predict that older adults will exhibit greater sensitivity to the fine-grained properties of test items than younger adults.
[These] patterns of performance reflect the information-processing costs that must inevitably be incurred as knowledge is acquired. Once the cost of processing this extra information is controlled for in studies of human performance, findings that are usually taken to suggest declining cognitive capacities can be seen instead to support little more than the unsurprising idea that choosing between or recalling items becomes more difficult as their numbers increase.