More exercise equals less brain shrinkage

The test involved around 600 people in Scotland who were given an MRI at age 70 then again at age 73. From LiveScience.com:

"People in their seventies who participated in more physical exercise, including walking several times a week, had less brain shrinkage and other signs of aging in the brain than those who were less physically active," said lead author Alan J. Gow of the University of Edinburgh in Scotland. "On the other hand, our study showed no real benefit to participating in mentally and socially stimulating activities on brain size, as seen on MRI scans, over the three-year time frame."

Of course there's the possibility that a shrinking brain makes a person less willing and able to exercise.