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Mental powers start to dwindle at 27 after peaking at 22, marking the start of old age, US research suggests.

Professor Timothy Salthouse of Virginia University found reasoning, speed of thought and spatial visualisation all decline in our late 20s.

Therapies designed to stall or reverse the ageing process may need to start much earlier, he said.

His seven-year study of 2,000 healthy people aged 18-60 is published in the journal Neurobiology of Aging.

To test mental agility, the study participants had to solve puzzles, recall words and story details and spot patterns in letters and symbols.

The first age at which there was any marked decline was at 27 in tests of brain speed, reasoning and visual puzzle-solving ability.

Things like memory stayed intact until the age of 37, on average, while abilities based on accumulated knowledge, such as performance on tests of vocabulary or general information, increased until the age of 60.