A new study makes the strongest pitch yet for exercising into your senior years.
Dr. Rich O'Neill, psycholotist at SUNY Upstate, says the study, just published by the National Academy of the Sciences randomly assigned 120 sedentary older men and women to walking 3 times a week for 40 minutes, or stretching and toning for the same time.  After a year, the walkers showed a 2% increase in the volume of their hippicampus, the part of the brain associated with memory.  The stretchers and toniers had a 1% loss in the hippocampus.  O'Neill says it's exercise, not aging, that affects memory loss we see with age.

To read the study (or a synopsis)  http://www.pnas.org/search?fulltext=exercise+training&submit=yes