Even Late in Life, a Low-Fat Diet Helps
The more fat you eat, the more likely you are to lose your marbles in your old age.
This graph came from an article that explains why it's reasonable to conclude that the fat in the diet is the culprit. It also explains that even late in life, a change to a better diet is beneficial.
The article mentions that in Europe and North America, higher fish consumption seemed to provide some reduction in risk. That may be because the fish were simply replacing foods that were even more dangerous. It doesn't mean that a health-optimizing diet for a human being would include fish.