The January 9 issue of Journal of Neuroscience published an article Lifelong Bilingualism Maintains Neural Efficiency for Cognitive Control in Aging (full article). Although there's no difference in "simple working memory span" between mono- and bi-linguals, "older adult bilinguals switched between perceptual tasks significantly faster than their monolingual peers." And "older adult bilinguals showed a pattern of fMRI results similar to the younger adult groups: they outperformed monolingual older adults while requiring less activation in several frontal brain regions linked with effortful processing." "[T]he bilingual requirement to switch between languages on a daily basis serves to tune the efficiency of language-switching regions..., and that over time the increased efficiency of these regions comes to benefit even nonlinguistic, perceptual switching".
It's no more news that multi-language efficiency or simply studying a foreign language postpones Alzheimer's onset. (In a leisure-reading article I wrote, I almost gave up on finding a reasonable excuse for my study of foreign languages and reluctantly settled on possible prevention of Alzheimer.) But it's easy to lose sight of news that hints at the negative side of bilingualism. For example, in Cognitive and Linguistic Processing in the Bilingual Mind (full article), published in February 2010 of Current Directions in Psychological Science, we read "bilinguals typically have lower formal language proficiency than monolinguals do; for example, they have smaller vocabularies and weaker access to lexical items."
Lastly, if bilingualism has the same effect as more education on delay of the onset of Alzheimer, be aware that research shows that in spite of delayed onset, faster progression of the disease after onset is associated with more education.
Nevertheless, there're far more benefits reported in research than possible harm in bilingualism to mental health. There may even be more, although possibly diminishing, benefit in trilingualism than bilingualism, but no research has been conducted to my knowledge. In a nutshell, cerebral stimulus as in language study is highly recommended in our natural aging and should be followed throughout the life, not to be terminated when you no longer need to take exams or when you retire.