An Australian study involving 102 older adults (60-90) has concluded that physical fitness and arterial stiffness account for a great deal of age-related memory decline.

The study that, while both physical fitness and aortic stiffness were associated with spatial working memory performance, the two factors affected cognition independently. More importantly, and surprisingly, statistical modelling found that, taking BMI and gender into account, fitness and aortic stiffness together explained a third (33%) of the individual differences in spatial working memory — with age no longer predicting any of the differences.

While physical fitness didn’t seem to affect central arterial stiffness, the researchers point out that only current fitness was assessed and long term fitness might be a better predictor of central arterial stiffness.

It's also worth noting that only one cognitive measure was used. However, this particular measure should be a good one for assessing cognition untainted by the benefits of experience — a purer measure of the ability to process information, as it were.

It would also be interesting to extend the comparison to younger adults. I hope future research will explore these aspects.

Nevertheless, the idea that age-related cognitive decline might be largely, or even entirely, accounted for by one's physical fitness and the state of one's arteries, is an immensely appealing one.

Fitness was assessed with the Six-Minute Walk test which involved participants walking back and forth between two markers placed 10 metres apart for six minutes. Only participants who completed the full six minutes were included in the analysis.

https://www.eurekalert.org/pub_releases/2018-06/ip-bpf061118.php