Via NYTimes.com:

Educators say that, for adults, one way to nudge neurons in the right direction is to challenge the very assumptions they have worked so hard to accumulate while young. With a brain already full of well-connected pathways, adult learners should “jiggle their synapses a bit” by confronting thoughts that are contrary to their own, says Dr. Taylor, who is 66.

Read more: http://www.bakadesuyo.com/2010/01/how-to-keep-an-aging-brain-sharp/