The brains of our closest relatives, unlike our own, do not shrink with age.
The findings suggest that humans are more vulnerable than chimpanzees to age-related diseases because we live relatively longer.
Our longer lifespan is probably an adaptation to having bigger brains, the team suggests in their Proceedings of the National Academy of Sciences paper.
Old age, the results indicate, has evolved to help meet the demands of raising smarter babies.
As we age, our brains get lighter. By 80, the average human brain has lost 15% of its original weight.
People suffering with age-related dementias, such as Alzheimer's, experience even more shrinkage.
This weight loss is associated with a decline in the delicate finger-like structures of neurons, and in the connections between them.
Alongside this slow decline in its fabric, the brain's ability to process thoughts and memories and signal to the rest of the body seems to diminish.
Researchers know that certain areas of the brain seem to fare worse; the cerebarl cortex, which is involved in higher order thinking, experiences more shrinkage than the cerebellum, which is in charge of motor control.
Yet despite the universality of ageing, scientists do not fully understand why our brains experience this continuous loss of grey matter with age.
Intriguingly, the brains of monkeys do not seem to undergo the same weight loss, begging the question of whether it is a distinctively human condition.