A new study has revealed that many well-known effects of ageing may start decades before the twilight years when old age is often blamed for causing many people to misplace keys, forget a word or lose the train of thought.

In fact, according to the researchers, people's mental abilities begin to decline from the age of 27 after reaching a peak at 22.

"Results converge on a conclusion that some aspects of age-related cognitive decline begin in healthy, educated adults when they are in their 20s and 30s," the 'Daily Mail' quoted lead researcher Prof Timothy Salthouse as saying.

They studied 2,000 men and women aged 18 to 60 over seven years to reach the conclusion.

The people involved -- who were mostly in good health and well-educated -- had to solve visual puzzles, recall words and story details and spot patterns in letters and symbols.

Similar tests are often used to diagnose mental disabilities and declines, including dementia.

The study found found that in nine out of 12 tests the average age at which the top performance was achieved was 22.

The first age at which performance was significantly lower than the peak scores was 27 -- for three tests of reasoning, speed of thought and spatial visualisation.

Memory was shown to decline from the average age of 37. In the other tests, poorer results were shown by the age of 42, the study by University of Virginia, found.