“You can't separate the mind and body,” Jack LaLanne once told USA Today.

LaLanne —sometimes called the Godfather of Modern Fitness — lived to be 96, and learned early in life that exercise (combined with proper nutrition) could make your body and mind both feel significantly younger.

In his heyday, critics called LaLanne a nut and a charlatan. But it turns out he was right.

A study in the journal Neurology reveals that sedentary folks 55 and older with cognitive impairment (difficulty concentrating, making decisions, or remembering) can turn back the clock nine years in just six months by doing aerobic exercise such as walking or cycling for 35 minutes three times a week.

Executive functioning, which refers to neurologically based skills involving mental control and self-regulation, was the feature that showed most notable improvement.

The very best cognitive improvements came for people who followed that exercise routine and the DASH (Dietary Approaches to Stop Hypertension) diet — a low-sodium, high-fiber nutritional plan that includes lots of fruits, vegetables, beans, nuts, low-fat dairy, 100 percent whole grains, and lean proteins.

While this wasn't a major study, it echoes the findings of another study from the University of Texas Southwestern Medical Center that suggested physical fitness is associated with stronger neuron fibers and better executive function in people with mild cognitive impairment.

Other research has found that aerobic exercise increases the volume of the hippocampus, the brain region responsible for memory.

So join an exercise group and have some fun. As George Bernard Shaw said: “We don't stop playing because we grow old; we grow old because we stop playing.”