The risk of dementia appears to be falling despite an aging population, U.S. doctors say.

In Wednesday's issue of the New England Journal of Medicine, doctors point to two recent U.S. studies and three from Europe suggesting declines in dementia incidence rates among people born in the first half of the 20th century.

Dementia After Dark

People with dementia do a parachute exercise in the Bronx. A new study shows the age of those getting dementia seems to be getting higher. (Jim Fitzgerald/Associated Press)

"For now, the evidence supports the theory that better education and greater economic well-being enhance life expectancy and reduce the risk of late-life dementias in people who survive to old age," Dr. Eric Larson, of the University of Washington in Seattle, and his co-authors said in their commentary.

They called the consistency of the findings "encouraging and noteworthy," given the projected growth of the population older than 75 years.

"Of course, people are tending to live longer, with worldwide populations aging, so there are many new cases of dementia," Larson said in a release.

"But some seem to be developing it at later ages — and we're optimistic about this lengthening of the time that people can live without dementia."

Earlier this year, the researchers reported in the same journal that people with lower blood sugar levels tend to have less risk of dementia. Focusing on greater physical activity, diet, educational opportunities both in early and later life , treating hypertension and quitting smoking also help prevent dementia, Larson said.

The researchers said the studies serve as a reminder that dementia is a syndrome with complex symptoms but that the vast majority of dementia cases, especially those occurring late in life, tend to involve a mixture of Alzheimer's disease and vascular disease.

British researchers who surveyed more than 7,500 people aged 65 and older between 1989 and 2011 also concluded that populations born later have a lower risk of dementia than those born earlier, probably because of higher education levels and better prevention of heart disease and stroke.

In 2010, a report commissioned by the Alzheimer Society of Canada suggested the prevalence of dementia in Canada will more than double in 30 years with the costs increasing 10-fold if no changes are made. One of the changes suggested is getting people over 65 to increase their physical activity levels.