In most people, their brains get smaller as they age. It is not so much that neurons die but that their terminals and synaptic junctions shrivel. A known cause is the over-secretion of cortisol by stress, but perhaps there are also other age-related causes.
However, shrinkage with age is not inevitable. Certain people are "super-agers," defined as adults over 80 with memory at least as good as normal middle-aged adults. A usually reliable index for decline in memory ability is the degree of brain shrinkage, specifically cortical volume. Brain-scan studies show that super-agers have thicker layers of cortex than do others of the same age. Thus, their cortex has not shrunk as much as average elderly or they had more to start with. It is possible that something about the lifestyle of super-agers protected them from brain atrophy. It is not convenient to know how much cortical volume the elderly had in their youth. But the second option has been tested in a study that compared the rate of cortical aging in 36 adults averaging 83 years of age. The investigators recruited super-agers and normal elderly and tested them in an initial visit and again 18 months later. Before and after cognitive and memory tests and brain scans provided a basis for tracking the rate of aging.
Super-agers scored higher on cognitive and memory tests than the average group at both the beginning and end of the study period. This suggests that they may have been endowed with more mental capability when they were young. But it also indicates that super-agers are more resistant to age-induced mental decline. The two groups did not differ in any other neuropsychological measures, education, or estimated IQ.
A clear correlation occurred between the two groups and cortical volume. The average memory group had over twice as much cortical shrinkage over the 18 months as did the super-agers. Some in the average group lost as much as 3.4% of cortical volume per year. If that continued over the next 10 years, they would suffer a devastating loss of over 30% in cortical volume.
Unfortunately, the study did not examine the lifestyles in the two groups. The super-agers may have just had good genes or may have been more mentally active over their lifetime and had healthier diets, more exercise, and less stress than those in the average group. Notably, some shrinkage did occur in the super-agers, on average at a rate of 1.06% per year. They still scored as well as the average 50-year old on various cognitive and memory tests. It is possible that some shrinkage is a good thing, reflecting perhaps a pruning of neural circuitry as the brain learns and develops more efficiency. Pruning is a conspicuous phenomenon in the brains of the fetus and infants as maturation progresses. Obviously too much pruning can leave neural circuitry with insufficient resources.
These results also emphasize that age discrimination is not defensible. Each elderly person's mental competence has to be judged on its own merits, not on a negative stereotype of the elderly.
Rogaalski,E. J. et al. (2013) Youthful memory capacity in old brains. J. Cognitive Neuroscience. 25(1), 29-36.
Cook, Amanda H. et al. (2017). Rates of cortical atrophy in adults 80 years and older with superior vs. average episodic memory. JAMA. 317(13), 1373-1375.