Muscle and joint stiffness is very common in older adults. It is not clear whether this occurs as a consequence of aging, inactivity, degenerative disease, adhesion (glue-like) molecules in the tissues, or a combination of all these things.
Having enough muscle bulk and strength is necessary for full joint motion. This requires healthy muscles and tendons with a good blood supply. Without these, raising the arms completely overhead becomes a challenge. Nutrition is important, and a strength-training program is recommended. Exercises will help offset the loss in muscle mass and strength typical of normal aging.
Discuss this situation with your doctor. It is important to make sure nothing more serious is causing these changes. Ask about seeing a physical therapist for a muscle-strengthening program. This can improve your movement and flexibility while preventing deconditioning. Taking these steps now can help prevent future injuries.