Canadian researcher published earlier this year a study showing that people who regularly used two languages showed signs of dementia 4 years later than people who used only one language!
It looks like bilingual brains are more resistant to the effects of cognitive decline. Previous studies by the same researchers had established that bilingual people were better at paying attention and ignoring distractions. Ellen Bialystok, the lead researcher on these studies, believes that this is because bilingual people always have to decide which language to use and suppress the other. This would provide practice in focusing attention, sorting through conflicting information and ignoring distraction.
Bilingualism may delay dementia in the same way that mental activity is thought to, that is by contributing to building cognitive brain reserve. However, we don’t know yet whether the same effects would appear if you start learning a language at 50.
Read the original journal article: Neuropsychologia, Volume 45, Feb 2007, pp.459-464