The body is a web of overlapping systems, many of which depend upon one another for effective function. If one system begins to weaken, so do many others. Degenerative aging is a global affair, occurring throughout the body, and so we should not be surprised to find strong correlations between specific forms of age-related decline in many different organs. That doesn't necessarily mean that there is anything profound hiding behind such an association - cells are accumulating damage in all tissues, body-wide systems such as blood vessel elasticity and the immune system are progressively failing, and so decline is everywhere:
Decreased kidney function is associated with decreased cognitive functioning in areas such as global cognitive ability, abstract reasoning and verbal memory. [This] is the first study describing change in multiple domains of cognitive functioning in order to determine which specific abilities are most affected in individuals with impaired renal function.
[Researchers] examined longitudinal data, five years apart, from 590 people. They wanted to see how much kidney function had changed over that time period, and whether it was associated with how much cognitive functioning had changed. They were interested in the overall change, but also in specific abilities such as abstract reasoning and verbal memory.
"The brain and kidney are both organs that are affected by the cardiovascular systems. They are both affected by things like blood pressure and hypertension, so it is natural to expect that changes in one organ are going to be linked with changes in another."