The Smithsonian's Surprising Science blog: 

The  findings, published online today in  Neurology, suggest that reading books, writing and engaging in other similar brain-stimulating activities slows down cognitive decline in old age, independent of common age-related neurodegenerative diseases. In particular, people who participated in mentally stimulating activities over their lifetimes, both in young, middle and old age, had a slower rate of decline in memory and other mental capacities than those who did not.

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Reading gives our brains a workout because comprehending text requires more mental energy than, for example, processing an image on a television screen. Reading exercises our working memory, which actively processes and stores new information as it comes. Eventually, that information gets transferred into long-term memory, where our understanding of any given material deepens. Writing can be likened to practice: the more we rehearse the perfect squat, the better our form becomes, tightening all the right muscles. Writing helps us consolidate new information for the times we may need to recall it, which boosts our memory skills.

Here are a couple of suggestions to get your reading workout going: Summer Reading: Independent Learning & The Novels of Max Barry.