For years, many experts have maintained that the subtle changes in memory and mental function that occur naturally as we get older rarely begin before age 60.

That may be optimistic: A new study, published Thursday in the British Medical Journal, suggests that age-related cognitive changes -- which may in some cases herald Alzheimer's disease and other types of dementia -- are under way as early as our mid-to-late 40s.

Researchers in Europe tracked the mental function of more than 7,000 British civil servants for a decade, and they found that even the youngest participants, who were between the ages of 45 and 49 at the outset, generally displayed slight yet measurable declines in short-term memory, mental reasoning, and verbal facility over the course of the study.

The declines were too small to be noticeable in everyday life and were detected only through a battery of tests the researchers gave the participants every three to four years. But the findings may have implications for the prevention of dementia, and underscore the importance of caring for our bodies and minds early in life, the researchers say.

"We, and others, have shown healthy lifestyles and good cardiovascular health to be important for cognitive outcomes," says lead author Archana Singh-Manoux, Ph.D., research director at the French National Institute of Health and Medical Research (INSERM), in Paris. "The fact that cognition declines early implies that midlife levels of these factors -- health behaviors and cardiovascular risk factors and disease -- might be important for cognitive outcomes later in life."

Indeed, years of research suggest that heart-healthy habits are also good for the brain. Although the results haven't always been consistent, previous studies have shown that obesity, unhealthy cholesterol levels, and a lack of exercise in midlife are all linked with an increased risk of dementia later on.

Brain-imaging studies, meanwhile, have found that abnormalities associated with cognitive decline, Alzheimer's, and other dementias can occur years -- even decades -- before any outward problems arise.

Researchers haven't conclusively proven that cognitive decline in middle age predicts Alzheimer's or other dementias, but on balance the evidence suggests that small changes in midlife mental function can become magnified later in life, says Francine Grodstein, Sc.D., an epidemiologist and associate professor of medicine at Brigham and Women's Hospital, in Boston.

"There is a lot of evidence that [people] with cognitive decline are at highest risk of later developing dementia, so it is likely that preventing or delaying cognitive decline today will help reduce risk of dementia tomorrow," says Grodstein, who was not involved in the research but wrote an editorial accompanying the study.

The researchers didn't look specifically at whether early mental decline was linked to dementia. Too few study participants reached old age during the study to provide reliable data on that score, but the researchers plan to continue following the same people in an attempt to answer that question, says Singh-Manoux, who is also a professor of epidemiology at University College London, in the U.K.

The new study -- the first of its kind to assess people as young as 45 -- is part of a larger, ongoing study known as Whitehall II. Beginning in the late 1990s, Singh-Manoux and her colleagues tested the mental function of 5,198 men and 2,192 women between the ages of 45 and 70 three times over a 10-year period.

In the short-term memory test, for example, the participants were shown 20 words for two seconds each and then had two minutes to write down as many as they could remember. Tests for verbal fluency included writing down as many animal names or "S" words as possible in a one-minute period.

After adjusting for education levels, the researchers found that scores in all areas except vocabulary dropped in all age groups during the study, with the declines accelerating as people got older. (They didn't expect to find any changes in vocabulary, since it generally isn't affected by age.)

On a test designed to measure verbal and mathematical reasoning, for instance, men who were 65 to 70 years old at the beginning of the study saw their scores decline by 10%, on average. On the same test, men and women ages 45 to 49 saw their scores drop by 3.6%. They experienced similar declines in other categories.

More research will be needed to confirm the findings, and to identify the factors that may contribute to early mental decline, the authors note. And the results may not apply to the population as a whole, since the participants were all stably employed government workers who were overwhelmingly white and had relatively high incomes.

Although it's far from the final word on the topic, the study is stronger in many ways than much of the previous research on early mental decline, says Gary Kennedy, M.D., director of geriatric psychiatry at Montefiore Medical Center, in New York City.

Despite their lack of ethnic diversity, the men and women in the study represent a relatively broad cross-section of the population, Kennedy says. By contrast, he says, brain-imaging studies often look only at people who have reason to suspect they already have cognitive decline.

And because the authors tracked the mental performance of individuals (rather than large groups) over time, the study results are less likely to be skewed by person-to-person differences in lifestyle or other risk factors. This makes the study "a little more worrisome and much more informative," Kennedy says.