Scientific studies of aged brains, particularly those impacted by dementia and other cognitive diseases, demonstrate that bilingualism and multilingualism protect the brain, somewhat. The study cited here notes the "focus on bilingual memory as an avenue to explore the relationship between executive control and cognitive reserve." A similar study of elderly adults found that "years of bilingualism appears to change how the brain carries out tasks, particularly those that require concentrating on one piece of information without becoming distracted by other information. This makes the brain more efficient and economical with its resources," (see Bilingual Brains).
These long-term benefits can begin in early childhood, as found by Mariano Sigman, who wrote The Secret Life of the Mind: How Your Brain Thinks, Feels, and Decides. Among other benefits, Sigman writes of how many studies have demonstrated that bilingual or multilingual brains exhibit better executive control. Such individuals are better able to focus, concentrate, and make connections between concepts, because of the language skills they have. Speaking more than one language allows them to think more efficiently. A recent study from Luxembourg found a similar impact on such individuals' ability to complete math problems. The study found that "brain regions associated with visual processing are activated in bilingual people when solving math problems in their second language—something that’s not seen in monolingual people, and which has lifetime implications."
-Mary Bridget Burns