Results found that compared to rare or no tea intake, the odds of cognitive impairment were 2/3 less in heavy tea drinkers (1/2 less in even low tea intake) compared to those who did not drink tea. No association was found between coffee intake and cognitive function, so coffee did not help. These results likely occur because tea (real brewed tea or teabags, not the powdered stuff) is high in antioxidants. Decaf tea loses 1/2 its antioxidants, and powdered tea and soda pop sold as tea has almost no antioxidant function left. Eating blueberries (also high in antioxidants) also improved mental function in another study.
"Tea consumption and cognitive impairment and decline in older Chinese adults," Ng TP, Feng L, et al, Am J Clin Nutr, 2008; 88(1): 224-31.