An Indian study involving 648 dementia patients, of whom 391 were bilingual, has found that, overall, bilingual patients developed dementia 4.5 years later than the monolingual ones. There was no additional advantage to speaking more than two languages.
The effect remained after factors such as education, sex, occupation, and urban vs. rural dwelling, had been accounted for. The finding is consistent with previous research, and is not only the largest study so far on the subject, but the first to show the effect also applies to illiterate people who had not attended school. Moreover, the effect was found in three different types of dementia: frontotemporal, vascular, and Alzheimer’s disease.
(2013). Bilingualism delays age at onset of dementia, independent of education and immigration status.
Neurology. 81(22), 1938 - 1944.