A cohort study of 2501 people from Singapore has found that regular tea consumption is associated with lower risks of cognitive impairment and decline. The effects were most evident for black (fermented) and oolong (semi-fermented) teas. In contrast, no association between coffee intake and cognitive status was found.

Tea consumption and cognitive impairment and decline in older Chinese adults. Am J Clin Nutr. 2008 Jul;88(1):224-31.