Exercise Your Brain, or Else You’ll ... Uh ...

When David Bunnell, a magazine publisher who lives in Berkeley, Calif., went to a FedEx store to send a package a few years ago, he suddenly drew a blank as he was filling out the forms.

“I couldn’t remember my address,” said Mr. Bunnell, 60, with a measure of horror in his voice. “I knew where I lived, and I knew how to get there, but I didn’t know what the address was.”

By 2050, according to the Alzheimer’s Association, 11 million to 16 million Americans will have the disease.