According to new research from the U.S., as people get older communication between different regions of the brain breaks down.
The scientists say this deterioration occurs even when serious conditions linked to mental decline, such as Alzheimer's disease are absent and is all part of the normal ageing process.
They also say such deterioration is likely to result in impaired memory and reasoning.
The scientists say the brain can be divided into major functional regions, each responsible for different processes such as memory, dealing with sensory inputs, planning and internal musings.
These regions are linked by a network of "white matter" nerve fibres, similar to the wiring in a computer and messages pass through the white matter to get from one region to another.
These communication channels help the co-ordination and sharing of information and though scientists know that white matter degrades with age, how that decline contributed to the degradation of the large-scale systems that govern cognition, was unclear.
The researchers from the Howard Hughes Medical Institute in Maryland, used advanced medical imaging techniques to carry out brain scans on 93 healthy people aged 18 to 93.
Participants who were older were also given a battery of tests to measure their mental abilities, including memory, "executive function" which governs overall planning and processing speed.
While the study revealed that in younger people their brain systems were largely "in sync" with each other, in older adults this was not so.
It was found that widely separated systems situated in the front and back of the brain were particularly out of step.
The researchers found that older individuals who performed better in the tests were more likely to have synchronised brains but they were not all affected the same way.
Professor Randy Buckner says the study shows that cognitive decline in ageing may be linked to the disruption of communication between different regions of the brain.
The researchers also say that other changes in the aging brain may contribute to cognitive decline.
The research is published in the current issue of the journal Neuron.