Contrary to popular belief, our brain does not necessarily shrink nor do our neurons short-circuit as we get older. Some of the decline people experience may be due not to aging, but to the cumulative impact of years of high blood pressure, clogged arteries, and repeated interruptions of blood flow to the brain. Some of the conditions that can dim your cerebral firepower are:

Alcoholism
Depression
Diabetes
Heart Disease
Sleep Apnea
Hypertension
Stress
Heart Surgery