The researchers looked at a population of 648 people in India. All had been diagnosed with some form of dementia. Their average age was 66.
When analyzing the data, they found that those who spoke two languages developed dementia about four and a half years later than those who spoke just one language.
The differences persisted whether they were able to read or not. Fourteen percent of those in the study were illiterate.
The later onset of memory loss, including Alzheimer's disease and vascular dementia, was also seen regardless of factors such as education, gender, occupation and rural or city residency.
"Our study is the first to report an advantage of speaking two languages in people who are unable to read," said study author Suvarna Alladi, with Nizam's Institute of Medical Sciences in Hyderabad, India.
This suggests "that a person's level of education is not a sufficient explanation for this difference," she said.
"Speaking more than one language is thought to lead to better development of the areas of the brain that handle executive functions and attention tasks, which may help protect from the onset of dementia."
There was no additional protective effect against dementia among people in the study who spoke more than two languages, said the study.