Running isn't just for the young and healthy - older runners with healthy joints also keep pounding the pavement.

Amanda Wagner and Jessica Tuttle both turn 50 this year, but they're not letting age keep them from running up to five days a week.

Researchers now say after 21 years of study that if you're healthy and generally injury-free, there are few reasons to put away your running shoes as you age, even into your 70s and 80s.

"Running in general is not bad for your knees or your joints as you age," Dr. R. Amadeus Mason of the Emory Sports Medicine Center said. "The biggest risk that runners will face as they age with regard to injury is over training by far."

Sports medicine experts also say healthy joints and muscles respond well to appropriate stressing, but the key is to listen to your body.