Senior moments, such as forgetting a recent conversation, may be a sign of the process that can lead to Alzheimer's disease, research suggests.
Many people find they start to forget as they get older
US scientists examined the brains of 134 older people who had appeared mentally sharp, apart from some subtle forgetfulness.
They found more than a third were riddled with the protein clumps associated with Alzheimer's.
The study is published in the journal Neurology.
Scientists are trying to identify the earliest point in the process that leads to Alzheimer's, as it is thought that treatment is most likely to be effective if it is given at the earliest possible stage.
Most people had assumed that minor episodes of forgetfulness are nothing to worry about.
The researchers found the brains of people in the study showed levels of deterioration similar to those found in patients with severe Alzheimer's.
Lead researcher Dr David Bennett, of Rush University Medical Center in Chicago, said the findings suggested some people were able to compensate for the damage much more successfully than others.
He said: "There is something about these people that allows them to have large amounts of pathology without obvious memory problems.
"We need to understand why that is."
Dr Bennett believes high levels of education may help to preserve brain function because it encourages more connections between brain cells.
Dr Carol Lippa, director of the memory disorders programme at Drexel University College of Medicine in Philadelphia, said the study questioned the widely held assumption that minor memory lapses in older adults were normal.
She said the findings seemed to back the theory that it was important to keep the brain as active as possible.
Rebecca Wood, chief executive of the Alzheimer's Research Trust, said: "For many years, scientists have seen examples of people who had no obvious clinical symptoms of dementia but have clear Alzheimer's pathology in the brain.
"Larger studies to investigate the extent of this in the wider population could help us to understand why this happens in some people and not others, and lead us in the search for new treatments."
Ms Wood said it was extremely difficult to distinguish between a healthy individual becoming a little forgetful and somebody in the early stages of Alzheimer's.
"There can be other reasons why somebody is experiencing mild memory problems, including illnesses such as depression, or medicines including anti-anxiety drugs, anti-depressants and drugs used to treat high blood pressure."
She said the trust was investigating other ways to diagnose Alzheimer's at an earlier stage, including possible blood or urine tests.