June Springer, who just turned 90, at Caffi Contracting Services where she works in Alexandria, Va.

New research boosts the "use it or lose it" theory about brainpower and staying mentally sharp. People who delay retirement have less risk of developing Alzheimer's disease or other types of dementia, a study of nearly half a million people in France found.

It's by far the largest study to look at this, and researchers say the conclusion makes sense. Working tends to keep people physically active, socially connected and mentally challenged — all things known to help prevent mental decline.

"For each additional year of work, the risk of getting dementia is reduced by 3.2 percent," said Carole Dufouil, a scientist at INSERM, the French government's health research agency.
She led the study and gave results Monday at the Alzheimer's Association International Conference in Boston.

About 35 million people worldwide have dementia, and Alzheimer's is the most common type. In the U.S., about 5 million have Alzheimer's — 1 in 9 people aged 65 and over. What causes the mind-robbing disease isn't known and there is no cure or any treatments that slow its progression.

France has had some of the best Alzheimer's research in the world, partly because its former president, Nicolas Sarkozy, made it a priority. The country also has detailed health records on self-employed people who pay into a Medicare-like health system.

Researchers used these records on more than 429,000 workers, most of whom were shopkeepers or craftsmen such as bakers and woodworkers. They were 74 on average and had been retired for an average of 12 years.

Nearly 3 percent had developed dementia but the risk of this was lower for each year of age at retirement.