The brain's capacity for memory, reasoning and comprehension can begin to decline from age 45, research has shown.
Previous studies have suggested 60 to be the age people experience a difference in their thinking abilities, but new research which studied more than 7,000 people over a 10 year period suggests differently.
Tasks were set to assess memory, vocabulary, hearing and visual comprehension skills. included recalling in writing as many words beginning with the letter S as possible and as many animal names as could be thought of.
The good news is that vocabulary didn’t wane at all between 45-60. However all other brain function performed poorly in the cognitive tests, which were measured 3 times over 10 years.
In men, there was a 3.6% drop in reasoning after 10 years 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 authors of study conducted by the Centre for Research in Epidemiology and Population Health in France and University College London in the UK concluded: "Cognitive decline is already evident in middle age (age 45-49)."
They added: "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."
Their research focused on civil servants aged between 45 and 70 at the start of cognitive testing in 1997 to 1999. The results will be published online in the British Medical Journal (BMJ).
Dr Anne Corbett, research manager at the Alzheimer's Society, said that although the study was useful, it didn’t show what caused dementia or what signs would show the decline in health.
"More research is now needed to help us fully understand how measurable changes in the brain can help us improve diagnosis of dementia.
"An early diagnosis is essential as it can provide access to support and potential treatments which can vastly improve people's quality of life."
Over 750,000 people in the UK have a form of dementia, more than half have Alzheimer’s disease. In just ten years a million people will be living with dementia. This will soar to 1.7 million people by 2051.
One in three people die from dementia in the UK.
Dr Simon Ridley, head of research at Alzheimer's Research UK, said: "It's important to note that the group studied here was not representative of the population as a whole, and it would be helpful to see similar studies carried out in a wider sample.
"Previous research suggests that our health in mid-life affects our risk of dementia as we age, and these findings give us all an extra reason to stick to our new year's resolutions.
"Although we don't yet have a sure-fire way to prevent dementia, we do know that simple lifestyle changes - such as eating a healthy diet, not smoking, and keeping blood pressure and cholesterol in check - can all reduce the risk of dementia."