Researchers at the University of California found that the brains of aged people find it difficult to switch back after a distraction.
And by the time they resume their original pattern of thinking, it could have gone from their heads, the Telegraph reported.
For the study, the researchers recruited 40 volunteers, half of whom were aged around
70 years while the other half were around 25 years.
Their brains were scanned while they were asked to memorise a picture of nature on a screen for 14 seconds. At random intervals a face was flashed up on the screen.
Each volunteer was then asked to describe the face and then recall the scene from nature.
The brain scans showed that activity in the brain switched during the distracting facial image in both volunteers -- but it happened much more slowly in the elderly.
They also had more problems recalling the original natural scene.
The researchers believe the slow switching affects short-term, or "working" memory -- the capacity to hold and manipulate information in the mind for a period of time.
Working memory is the basis of all mental operations, from learning a friend's telephone number, and then entering it into a smart phone, to following the train of a conversation, to conducting complex tasks such as reasoning, comprehension and learning.
Professor Adam Gazzaley, a neurologist and co-author, said: "The impact of distractions and interruptions reveals the fragility of working memory.
"This is an important fact to consider, given that we increasingly live in a more demanding, high-interference environment, with a dramatic increase in the accessibility and variety of electronic media and the devices that deliver them, many of which are portable.
"Our findings suggest the negative impact of multitasking on working memory is not necessarily a memory problem, per se, but the result of an interaction between attention and memory."
His team is now exploring the potential of software brain-training programs to help older people improve their ability to mentally process tasks simultaneously.
The research was published in the Proceedings of the National Academy of Sciences.
