Neuropsychologists have recently found that the added cognitive reserves gained from speaking in more than one language for a prolonged period of time can help greatly with the onset of Alzheimer's Disease and other forms of mental dementia for an average of 4 years. This is not to say that those that bilingual speakers are more intelligent, able to avoid dementia entirely, or have more resilient brains than monolingual people - the ability to speak another language is likened to a 'reserve tank': once your brain runs out of 'fuel', your reserves can simply help you to carry on a little further.
Bilingualism doesn't delay dementia - it simply provides the brain with that 'reserve tank' to find ways to deal with the early symptoms of the slowing down of one's mental faculties. It is similar to the mental gymnastics of doing a daily crossword or Su Doku puzzle, or anything to keep the brain active - though there is no hard evidence to determine whether this does in fact help stave off dementia.
From the article:
Dr. Bialystok began her decades-long research by studying how children learn a second language.
In 2004, she and her colleague Fergus Craik shifted to conduct three studies looking at the cognitive effects in some 150 monolingual and bilingual people between 30 and 80 years old. They found that in both middle and old age, the bilingual subjects were better able to block out distracting information than the single-language speakers in a series of computerized tests. The advantage was even more pronounced in the older subjects.
Dr. Bialystok says other research also shows better performance from bilingual people on tests requiring cognitive control, such as when they are instructed to determine whether a sentence is grammatically correct, even if the content doesn't make sense. For example, in distinguishing, "apples grow on trees" from "apple trees on grow" and "apples grow on noses," the third sentence requires people to focus on the structure and suppress paying attention to the meaning of the words.
The findings from the 2004 study led Dr. Bialystok to wonder whether these benefits might help older people compensate for age-related losses in learning.
She and her colleagues examined the medical records of 228 memory-clinic patients who had been diagnosed with different kinds of dementia, two-thirds with Alzheimer's disease. The results, published in the journal Neuropsychologia in 2007, suggested that bilingual patients exhibit problematic memory problems later than those who only spoke one language.
Bilingual patients were, on average, four years older than single-language speakers when their families first noticed memory problems, or when the patient first came to the clinic seeking treatment.
So, all you bilingual speakers out there - it really helps to use both your languages regularly. Your future self will thank you one day!