They owe their quick wits to a lack of fibrous tangles which normally make an appearance in the brain late in life.
The accumulating protein tangles, known as "tau", are thought to kill off neurons, leading to poor memory.
Moderate numbers of them are found in the brains of most elderly people, and much larger numbers in Alzheimer's patients.
But somehow the brains of mentally nimble octogenarians are protected against tangles.
Scientists made the discovery after examining the brains of five deceased people who performed remarkably well in memory tests when they were more than 80 years old.
Most people have an elderly grandparent or aunt who puts youngsters to shame with their quiz skills or ability to rattle through crossword puzzles.
Professor Changiz Geula, from Northwestern University in Chicago, wondered if there was something about their brains that set them apart.
To find out, he recruited a group of "super-aged" individuals and subjected them to memory tests.
Some of them carried out the tasks as well as 50-year-olds. For example, after being told a story they were able to remember it immediately and accurately recall its details 30 minutes later. They could also memorise a list of 15 words and recall them after 30 minutes.
When five of the volunteers died, their brains were dissected and examined by the researchers.
They were found to contain far fewer tau tangles than the average brain of someone over 80.
"This new finding in super-aged brains is very exciting," said Prof Geula, from the Cognitive Neurology and Alzheimer's Disease Centre at the university's Feinberg School.
"It was always assumed that the accumulation of these tangles is a progressive phenomenon through the ageing process. But we are seeing that some individuals are immune to tangle formation and that the presence of these tangles seems to influence cognitive performance."
Individuals with few tangles performed better in memory tests, he said.
The research, presented at the Society for Neuroscience's annual meeting in Washington DC, is part of a larger "super-ageing" study taking place at Northwestern University.
The investigation's aim is to identify high-functioning people over the age of 80 and see what factors contribute to their quick wits.
Professor Clive Ballard, director of research at the Alzheimer's Society, said: "Dementia is caused by diseases of the brain and robs people of their lives. It is not a natural part of ageing, but age is the biggest risk factor.
"This small study may be a stepping-stone to further research that helps us understand why some people retain good cognitive function in later life.
"This research suggests that older people with good memory do not experience the same brain changes that develop in those with poor memory.
"Further research can build upon these findings and look specifically at the brain characteristics which enable people to stay healthy and sharp in later life."