This finding runs counter to conventional wisdom that mental decline doesn't begin before 60, the researchers added.
"Cognitive function in normal, healthy adults begins to decline earlier than previously thought," said study author Archana Singh-Manoux.
"It is widely believed that cognitive ability does not decline before the age of 60. We were able to show robust cognitive decline even in individuals aged 45 to 49 years," added Singh-Manoux, research director at INSERM's Center for Research in Epidemiology & Population Health at the Paul-Brousse Hospital in Paris.
These findings should be put in context of the link between cognitive function and the dementia, Singh-Manoux said.
"Previous research shows small differences in cognitive performance in earlier life to predict larger differences in risk of dementia in later life," she said.
Understanding cognitive aging might enable early identification of those at risk for dementia, Singh-Manoux said.
The report was published in the Jan. 5 issue of BMJ.
For the study, Singh-Manoux and colleagues collected data on nearly 5,200 men and 2,200 women who took part in the Whitehall II cohort study. The study, which began in 1985, followed British civil servants from the age of 45 to 70.
Over 10 years, starting in 1997, the participants' cognitive function was tested three times. The researchers assessed memory, vocabulary, hearing and vision.
Singh-Manoux's group found that over time, test scores for memory, reasoning and vocabulary skills all dropped. The decline was faster among the older participants, they added.
Among men aged 45 to 49, reasoning skills declined by nearly 4 percent, and for those aged 65 to 70 those skills dropped by about nearly 10 percent.
For women, the decline in reasoning approached 5 percent for those aged 45 to 49 and about 7 percent for those 65 to 70, the researchers found.
"Greater awareness of the fact that our cognitive status is not intact until deep old age might lead individuals to make changes in their lifestyle and improve [their] cardiovascular health, to reduce risk of adverse cognitive outcomes in old age," Singh-Manoux said.
Research shows that "what is good for the heart is good for the head," which makes living a healthy lifestyle a part of slowing cognitive decline, she said.
Targeting patients who have risk factors for heart disease such as obesity, high blood pressure and high cholesterol might not only protect their hearts but also prevent dementia in old age, the researchers said.
"Understanding cognitive aging will be one of the challenges of this century," especially as people are living longer, they added.
In addition, knowing when cognitive decline is likely to start can help in treatment, because the earlier treatment starts the more likely it is to be effective, the researchers noted.
Francine Grodstein, an associate professor of medicine at Brigham and Women's Hospital in Boston and author of an accompanying editorial, said more research is needed into how to prevent early cognitive decline.
"If cognitive decline may start at younger ages, then efforts to prevent cognitive decline may need to start at younger ages," she said.
"New research should focus on understanding what factors may contribute to cognitive decline in younger persons," Grodstein added.
"This is consistent with what we have seen in other studies and the cognitive changes that occur as we age," said Heather M. Snyder, senior associate director of medical & scientific relations at the Alzheimer's Association.
These changes do not mean that all these people will go on to develop Alzheimer's disease or another dementia, Snyder noted. "It is important to remember that the cognitive changes associated with aging are very different from the cognitive changes that are associated with Alzheimer's disease," she stressed.
Although some of these people may go on to develop Alzheimer's disease there is currently no way to tell who is at risk, Snyder said. "This is why it is so important to continue to investigate biological changes that occur in the earliest stages, because it is difficult to [determine] the cognitive changes that are associated with Alzheimer's disease," she said.
Snyder noted that Alzheimer's disease can start 15 to 20 years before symptoms are apparent, which makes finding a biological marker so important. "If a therapeutic is available, we can intervene at that point," she said.
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