An 11,000 person study by researchers at The University of Western Ontario showed that bilingualism conferred no general cognitive advantages across a dozen standardized tests. Other, earlier studies have had conflicting results, but were smaller.

Another issue examined in the study, was the frequent assertion that bilingualism provided protection from age-related cognitive decline. Unfortunately, this study provided no support for such protectitive effects.

Bottom-line: Becoming bi- or multi-lingual is valuable for the additional communications capabilities the additional languages provide. But don't do it because you think it will improve other cognitive abilities, or prevent possible age-related cognitive decline.