From New Scientist:
People who are fully bilingual and speak both languages every day for most of their lives can delay the onset of dementia by up to four years compared with those who only know one language, Canadian scientists said on Friday.
Researchers said the extra effort involved in using more than one language appeared to boost blood supply to the brain and ensure nerve connections remained healthy – two factors thought to help fight off dementia.
“We are pretty dazzled by the results,” Professor Ellen Bialystok of Toronto’s York University said in a statement.
More here.