A study of 2,282 people aged 18 to 87 found that hitting 40 was synonymous with forgetfulness, lack of concentration and poor focus. While general intelligence appeared to remain stable over time, psychologists concluded that everyday mental skills, such as remembering a telephone number or a person's name, showed a marked decline from the age of 40 onwards.

Prof Keith Wesnes, from Cognitive Drug Research Ltd, a private research company in Reading, discovered that people aged between 40 and 50 were 15 per cent slower at completing simple computerised tasks than those in their twenties. He told the British Psychological Society's conference in London: "The decline is most strongly associated with speed rather than accuracy.

"People in their forties clearly get worse at remembering, recalling and recognising things. This can be very embarrassing when someone walks into a busy room and can't remember people's names. At this age you can't concentrate as well, and you can't focus and ignore distractions to the same extent that you used to."

The computerised tasks performed by the volunteers included tests of their reaction time, their memory of words, pictures and digits, and their ability to maintain concentration. What causes the decline in memory remains uncertain. One theory is that the problem is linked to the deterioration of the messaging system in the brain.

It is understood that as people get older their brain cells lose some of their ability to communicate with each other via chemicals called neurotransmitters. But researchers insisted that there was no cause for alarm. Dr James Semple, a brain specialist at Addenbrooke's Hospital in Cambridge, said that with age came experience, and a series of defence mechanisms.

He said: "Old dogs have strategies that they can bring to mind and draw on to compensate for the loss of speed. Older people may be slowing down but they have years of experience that they can rely on." Clinical trials have showed that herbal remedies can help to improve memory among older people.

A recent study found that the memory of people aged between 40 and 65 improved by 7.5 per cent after taking supplements containing ginkgo and ginseng.