An inability to ignore distractions is the main reason why older people have memory problems, research suggests.
The study examined the brains of adults aged from 19 to 77
A team at the University of California, Berkeley, used scans to examine the brain's ability to concentrate in adults aged 19 to 30 and 60 to 77.
They found the older people had no problems focusing on relevant information - but could not effectively shut out competing distractions.
The research appears in the journal Nature Neuroscience.
Lead researcher Dr Adam Gazzaley said: "Difficulty filtering out distractions impacts a wide range of daily life activities, such as driving, social interactions and reading, and can greatly affect quality of life.
"These results reveal that efficiently focusing on relevant information is not enough to ensure successful memory.
"It is also necessary to filter distractions. Otherwise, our capacity-limited short-term memory system will be overloaded."
His colleague Professor Mark D'Esposito said: "If you are unable to block out distracting information, you can't really attend to what you are supposed to attend to, you can't get in what you are supposed to remember, and you have a hard time retrieving what you are supposed to remember."
The researchers are now examining whether a drug used to treat Alzheimer's disease could help solve the problem.
The researchers compared adults aged 19 to 30 with adults aged 60 to 77 using a simple memory test that introduced irrelevant information.
The tests were conducted while subjects' heads were inside a sophisticated scanner so that activity in the brain could be pinpointed.
While young subjects were easily able to suppress brain activity in areas that process information irrelevant to the memory task, older adults on average were unable to suppress such distracting information.
Both groups were equally able to enhance brain activity in the areas dealing with information relevant to the task.
Interestingly, six of the 16 older adults had well-preserved short-term memory and no problems ignoring irrelevant information - suggesting some people are able to avoid memory loss as they age.
The researchers hope to find out what makes these people different from the average ageing adult.
Professor Ian Robertson, dean of research at the Trinity College Institute of Neuroscience, said: "Given that the frontal lobes are important for selective attention and reducing distractibility, then this finding is understandable and important.
"I am sure memory lapses will be treatable by drugs in the relatively near future."
Harriet Millward, of the Alzheimer's Research Trust, said: "All of the individuals who volunteered for this study were healthy and so this research does not prove whether this is also a problem seen in patients with Alzheimer's disease and other dementias.
"Further research involving people diagnosed with cognitive impairment or Alzheimer's disease would be of interest to see whether this is a factor in dementia patients."