As time progresses, researchers are increasingly able to correlate changing mental characteristics in aging with changing structure in the brain. Here is one example: "A brain-mapping study [has] found that people's ability to make decisions in novel situations decreases with age and is associated with a reduction in the integrity of two specific white-matter pathways that connect an area in the cerebral cortex called the medial prefrontal cortex with two other areas deeper in the brain. Grey matter is the part of the brain that contains the bodies of the neurons while white matter contains the cable-like axons that carry signals from one part of the brain to another. In the past, most brain-imaging research has concentrated on the grey matter. Recently, however, neuroscientists have begun looking more closely at white matter. It has been linked to the brain's processing speed and attention span, among other things, but this is the first study to link white matter to learning and decision making. ... The evidence that this decline in decision-making is associated with white-matter integrity suggests that there may be effective ways to intervene. Several studies have shown that white-matter connections can be strengthened by specific forms of cognitive training. ... The critical white-matter connections that the experiment identified run from the thalamus, a highly connected relay center in the brain, to the medial prefrontal cortex, an area of the brain involved with decision making, and from the medial prefrontal cortex to the ventral striatum, which is associated with the emotional and motivational aspects of behavior." You might also look at past research on age-related damage to white matter and its consequences on metal capacity.