Researchers found people who were in their 80s when they took thinking and memory tests in the late 2000s performed similarly to others who were tested more than 10 years earlier while in their 70s.
General health in old age is probably improving for most people, Dallas Anderson said.
"People are better educated than they used to be, their economic wellbeing may be better compared to previous groups," Anderson said. He studies dementia at the National Institute on Aging in Bethesda, Maryland, and was not involved in the new study.
"All these various factors working together lead to an improved situation."
For their study, researchers tested the thinking and memory skills of 204 elderly French men and women selected from the memory clinic of a Paris hospital between 1991 and 1997. They compared their test scores to those from 177 similar people tested at the same clinic in 2008 and 2009.
None of the participants had dementia at the time.
As expected, people under age 80 performed better on the cognitive tests than older participants during both study periods, researchers led by Jocelyne de Rotrou from Hôpital Broca in Paris wrote in PLOS One.
The 2000s group as a whole also did better than the 1990s group. Participants tested more recently scored an average 83.2 out of a possible 100 on the exams, compared to 73.5 for their earlier counterparts.
The differences were consistent across almost every component of the tests, including how well people remembered stories and pictures and their ability to separate objects into different categories.
The authors said this trend might simply parallel increased life expectancies: the longer you live, the more good years you have.
But there could be something else going on too, Anderson told Reuters Health, like improvements in the average person's education and socioeconomic status.
These new results may also indicate that better drug regimens for controlling blood pressure and heart problems are having a positive effect on aging, he said.
"This is consistent with other studies, especially a couple already published from Europe, in Denmark and the UK," and it's likely happening in the U.S. as well, he said.
But Louis Bherer, who studies cognitive decline at Concordia University in Montreal, Quebec, believes it is too soon to generalize the Paris results more broadly.
"France is a country in which education is free and open to everyone, and where everyone has access to free medical care," he told Reuters Health. "Generalization to countries that do not offer the same social advantages would be misleading."
It's possible that more careful screening for early stages of dementia in 2008 and 2009 led to a more mentally sound group, said Bherer, who didn't participate in the new research.
As often happens, it's hard to tell whether researchers are sensing a true trend in the population, or tools have improved and changed what they can see.
Bherer and Anderson agreed these types of studies need to be replicated before any larger conclusions can be drawn.
A recent editorial in the New England Journal of Medicine highlighted evidence of declining dementia rates in the U.S.
"The idea that some people will get extra years of healthy living before they get demented, that's important," Anderson said. "When you look at it from a public health perspective, it's huge."
But the trend might not continue, he said, especially in the U.S. as more obese, diabetic generations age into retirement. Their health problems could help speed mental decline.
Dementia is still a public health issue, especially with the baby boomer generation getting older, he said.
"Even if the rates go down, the numbers are still going to go up."