The obligatory link: Bilingualism has protective effect in delaying onset of dementia by four years, Canadian study shows
It's an interesting idea, I suppose. Of course, they don't say what constitutes real "lifelong use of two languages". Does picking up a second language at seventeen count? (And if it does, does that make me an honorary member of the juunana sai cult?)
Looking down the article, it takes better than half the article to finally get to the real statistical value of the study... a whole 184 people. So, file this one under "more research is needed", I suppose.