By age 80, the brain will have lost, on average, 7 percent of its peak weight. As the brain ages, it loses neurons, and the speed of some types of thinking can slowdown. Uncorrected hearing loss and failing vision also impair mental alertness.
However, the older brain compensates by increasing its "crystalline intelligence," or the ability to solve problems by using previously acquired knowledge and solutions. Experience proves to be the best teacher and something the aged clearly have more of than the young.