Sudden increases in activity can place extensive stress on the shoulders and lead to a decrease in flexibility. This is a common problem in middle age, especially among "weekend warriors," or people who don't exercise regularly but go out every now and then for an intense sport.

Although painful and inconvenient, these overuse problems can usually be treated with rest, NSAIDs and stretching exercises.

Arthritis

Beginning as early as age 50, some people develop osteoarthritis, which causes painful movement. This occurs as the smooth surfaces of the cartilage that line the bones of the shoulder joint are worn away, and joints begin to wear out and become larger. The most common cause of osteoarthritis is overuse. Treatments for arthritis in the shoulder depend on the severity of pain. The usual treatments are rest, NSAIDs and cortisone injections.  In some instances, a replacement of the shoulder joint is necessary.

Source: http://resources.purematters.com/healthy-body/joint-tissue-health/common-injuries-of-the-shoulder