Mental slowing down in old age can be blamed partly on being more easily distracted, research suggests.
The Canadian team asked young and old people to attempt a memory test while in a scanner showing which bits of their brain were working.
The older subjects did worse at the tests, and their brains responded more to the background buzzing and banging from the scanner itself.
The study was published in the Journal of Neuroscience.
Other researchers have suggested that mental decline may be due to a decreasing ability to "tune out" irrelevant information from their senses.
This has been shown for both sound and vision, where older subjects were more likely to focus on the landscape in a picture rather than the figure within it.
The University of Toronto study used a standard face recognition test, placing 12 old and 12 young volunteers within a "functional MRI" scanner, which allows scientists to see which parts of the brains are activated during a particular activity.
Of prime interest was activity in the hippocampus, and area of the brain known to be involved with the laying down of memories.
When both the old and young volunteers failed to remember a face, there was less activity in the hippocampus, as might be expected.
However, when the older subjects failed, there was also increased activity in two other parts of the brain, the auditory cortex and the pre-frontal cortex, which are responsible for processing signals from the external environment.
Dale Stevens, who led the study, said that that brains of the older people were processing too much unnecessary information - in this case the normal knocking and rattling sounds that an MRI machine produces.
"The old brains showed increased activation in certain regions that should normally be quieter or turned down."
He said that the poorer performance overall of the older people might be due to an inability to "tune out" this noise while their brains were trying to form new memories of the faces.
Dr Jan de Fockert, a psychologist from the Institute of Cognitive Neuroscience at University College London, said that there were a number of competing theories as to why "selective attention" - the ability to focus on something in spite of distraction - might decline with age.
One suggestion, he said, is that the brain loses the ability to inhibit the processing of distracting signals.
Another suggests that an age-related decline in "working memory" - the information storage needed for the completion of everyday tasks, rather than longer-term storage - made it harder for individuals to pay close attention to one thing.
He said: "This seems like a very interesting paper, although it does not prove that the problems with selective attention are contributing to poor performance in the memory tasks."