The shoulder joint is made up of a ball and socket joint that on its own is inherently unstable as the socket component is very shallow. This allows the arm to have a huge range of motion in everyday life but because of this requires additional supporting structures to help stabilize and move the arm.
This makes the shoulder joint prone to injury at all ages of life. In younger patients the shoulder can be injured through sporting activities, the middle to older age groups the supporting muscles and tendons can become weaker and cause pain. In the older age groups like all cartilage joints it can wear out resulting in arthritis.