People who speak more than one language and who develop dementia tend to do so up to five years later than those who are monolingual, according to a study.
Scientists examined almost 650 dementia patients and assessed when each one had been diagnosed with the condition.
They found people who spoke two or more languages experienced a later onset of Alzheimer's disease, vascular dementia and frontotemporal dementia.
The bilingual advantage extended to illiterate people.
The scientists said it confirmed the observed effect was not caused by differences in formal education.
The study was by Edinburgh University and Nizam's Institute of Medical Sciences in Hyderabad in India.
It is the largest study so far to gauge the impact of bilingualism on the onset of dementia, independent of a person's education, gender, occupation and whether they live in a city or in the country, all of which have been examined as potential factors influencing the onset of dementia.
The team of researchers said further studies were needed to determine the mechanism, which causes the delay in the onset of dementia.
The researchers suggested bilingual switching between different sounds, words, concepts, grammatical structures and social norms constituted a form of natural brain training, which was likely to be more effective than any artificial brain training programme.
However, studies of bilingualism are complicated in that bilingual populations are often ethnically and culturally different from monolingual societies.
In places like Hyderabad, bilingualism is part of everyday life, knowledge of several languages is the norm and monolingualism is an exception.
Thomas Bak, of Edinburgh University's school of philosophy, psychology and language sciences said: "These findings suggest that bilingualism might have a stronger influence on dementia than any currently available drugs.
"This makes the study of the relationship between bilingualism and cognition one of our highest priorities."
The study is published in Neurology, the medical journal of the American Academy of Neurology.