The brains functioning can slow down around 15 years earlier than first believed, according to a new 10-year study.
Scientists from the Centre for Research in Epidemiology and Population Health in France and University College London, said mental function can start to decline as early as 45.
It was widely thought 60 was the age when people’s thinking abilities started to change.
The study looked at more than 7,000 people over a decade, with research mainly focusing on civil servants aged between 45 and 70 at the start of cognitive testing in 1997 to 1999.
In men, there was a 3.6% drop in reasoning over the decade, among those who were aged 45 to 49 at the start of the study and 9.6% among those aged 65 to 70.
In women, the decline was 3.6% and 7.4% in the same age groups respectively.
The study, which has been published in the British Medical Journal (BMJ), said: “Cognitive decline is already evident in middle age (age 45-49).”
“Life expectancy continues to increase, and understanding cognitive ageing will be one of the challenges of this century.
“Poor cognitive status is perhaps the single most disabling condition in old age.”