Alzheimer's may be associated with old age, but it seems that the brain's ability to function can start to decline much earlier.
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According to a new study published in the British Medical Journal, our mental performance starts to go downhill in our mid 40s.
The research, carried out by University College London, looked at the memory, vocabulary and comprehension skills of 7,000 men and women aged 45 to 70 over 10 years.
They found a 3.6% decline in mental reasoning in women and men aged 45-49 – which is much younger than previously thought. Previous studies had suggested that the decline didn't start until around the age of 60.
For those aged 65-70 there was a 9.6% decline in mental reasoning for men and a 7.4% decline for women.
The findings are important, say researchers, as treatments for dementia work best when started at the first sign of a problem.
Professor Archana Singh-Manoux, who led the research, warned that rates of dementia were "going to soar" due to poor lifestyle choices, such as smoking and lack of physical activity, which have both been linked to a decline in cognitive function.
"It's important to identify the risk factors early. If the disease has started in an individual's 50s but we only start looking at risk in their 60s, then how do you start separating cause and effect?" he said.
Dr Simon Ridley Alzheimer's Research UK said: "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."