Old age is often blamed for causing us to misplace car keys, forget a word or lose our train of thought.

But new research shows that many well-known effects of ageing may start decades before our twilight years.

According to scientists, our mental abilities begin to decline from the age of 27 after reaching a peak at 22.

The researchers studied 2,000 men and women aged 18 to 60 over seven years. 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 research at the University of Virginia, reported in the academic journal Neurobiology Of Aging, 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.

Professor Timothy Salthouse said the results suggested that therapies designed to prevent or reverse age-related conditions may need to start earlier, long before people become pensioners.

He wrote: ‘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.’

There is some good news, though. The report states that abilities based on accumulated knowledge, such as performance on tests of vocabulary or generalinformation, increase until at least the age of 60.