There are plenty of good reasons to exercise. In addition to managing weight and stress, and preventing the risk of several diseases, researchers now find increasing evidence that physical activity keeps the mind sharp and the brain healthy.
The idea that fitness can change the brain's physiology didn't catch on until recently, when technological advances in MRI allowed scientists to examine the brain on a molecular level. Exercise has been found to increase the development of new brain cells, which are accompanied by the growth and increase in the quality of the neurons. New growth of cells also produces a growth of new vascular capillaries, which provides more oxygen-rich blood to the brain. Aerobic activity has also produced a number of growth factors like insulin-growth factor-1 and and brain-derived neurotrophic factor that may grow and repair the brain. Physical fitness actually shifts where the blood flows in the brain specifically preserving brain cells associated with cognitive function.
Even though the way in which exercise affects the brain is still a mystery, it has been proven that staying physically fit keeps the brain strong and healthy. It's never too late; fitness in middle age and beyond reduces the biological effects of aging and any further declines in cognitive function. In some cases, function has shown improvement. Swimming, walking, dancing, bicycling, and aerobic classes are examples of "moderate" exercise. Getting some mild to moderate exercise daily is the best thing you can do for yourself, because life is worth it.
~National Review of Neuroscience, 2008