As a physical therapist, Nick Theders is a specialist in high school and Olympic-level swimmers and water polo players, and works on their shoulder and knee injuries.

But, in reality, most of the patients he sees at his practice, Think Physical Therapy in Tustin, Calif., are older adults with shoulder and knee injuries - or other problems associated with orthopedic conditions.

In the summer, his practice also includes weekend athletes who haven't played or worked out for years but have hurt their knees or shoulders or backs after joining the grandkids in a pickup game.

"They end up with rotator cuff injuries, tendinitis...