Thomas Bak, of the University of Edinburgh and colleagues at the Nizam's Institute of Medical Sciences in Hyderabad, India, examined almost 650 dementia patients and assessed when each one had been diagnosed with the condition.
The study, published in Neurology, found people who spoke two or more languages experienced a later onset of Alzheimer's disease, vascular dementia and fronto-temporal dementia -- and this bilingual advantage extended to illiterate people who had not attended school.
The finding was independent of a person's education, gender, occupation and whether they lived in a city or in the country, all of which have been examined as potential factors influencing the onset of dementia, Bak said.
Studies of bilingualism are complicated because bilingual populations are often ethnically and culturally different from monolingual societies. However, in places such as Hyderabad, bilingualism is part of everyday life and knowledge of several languages is the norm and monolingualism an exception, Bak said.
"These findings suggest that bilingualism might have a stronger influence on dementia that any currently available drugs," Bak said in a statement. "This makes the study of the relationship between bilingualism and cognition one of our highest priorities."