Heather recently reviewed a book called Organizing for Your Brain Type. According to this article, it might well have been called Organizing for Your Brain. It seems that "consciencious" people—orderly, dependable, hard-working, goal-oriented, self-disciplined, organized folks—are at significanly less risk of developing Alzheimer's and other forms of dementia as they age.
Autopsies revealed that these people did, in fact, have the kind of brain damage associated with Alzheimer's. What made the difference, apparently, was their ability to cope despite the damage.
So there you have it. Get organized, keep your promises, do good work. It's not only good for your soul, it's good for your brain.
In old age, conscientiousness seems to have to do a lot with the risk of dementia. It’s not that it directly affects the underlying pathology. It seems to affect your ability to tolerate the pathology and maintain normal cognitive function.