Supplementation with omega-3 fatty acids does not help prevent age-related cognitive decline and dementia, according to an article at MedPage Today.
The respected Cochrane organization did a meta-analysis of three pertinent studies done in several countries (Holland, UK, and ?).
The investigators leave open the possibility that longer-term studies—over three years—may show some benefit.
I leave you with a quote from the MedPage Today article:
And while cognitive benefits were not demonstrated in this review, Sydenham and colleagues emphasized that consumption of two servings of fish each week, with one being an oily fish such as salmon or sardines, is widely recommended for overall health benefits.