Study leader Carole Dufouil, a scientist at the National Institute of Health and Medical Research, a biomedical and public health research institution in France, said the study involved 429,000 people, with an average age of 72 and who were retired for an average of 12 years.
The researchers analyzed mental states of the study subjects as their retirement progressed. The subjects' records were obtained via the country's filing system for all self-employed people who pay into a Medicare-like health system. Many of the participants were shopkeepers or craftsmen, such as bakers or woodworkers, Medical Daily reported.
The team found 3.2 percent had developed dementia, but the rate of disease decreased as people aged in retirement.
For example, someone who retired at 65 had about a 15-percent lower risk of developing dementia compared to someone who retired at 60, after other factors were taken into account, Dufouil said.
The findings were presented at the Alzheimer's Association International Conference in Boston Monday.