Our shoulder joint is a ball and socket joint, like our hip joint.
Back (posterior) view Front (anterior) view
Back (posterior) view of hip Side (lateral) view of hip
As you can see the shoulder joint has a shallower socket as this allows greater range. The down side to the shallower socket is that all the connective tissue around the joint is at a higher risk of injury and damage. Shoulder range can decrease over the years because that connective tissue starts to tighten up. This means the body then compensates for the lack of range at the joint by using other parts of the body to basically get the hand where the brain says it wants it to go. This compensatory movement then creates other issues throughout the body.
Here is one quick and easy way to start to work on getting that connective tissue to give a little bit and therefore start to increase the range of movement back at the shoulder joint.
Do this 3-4 times each side and check in with your shoulder range. If one side is in worse shape than the other, then do a few more reps. Don't over do it. If your connective tissue is really stuck then doing too much too soon will actually set you back. No different to doing too much exercise too soon. You have to give the connective tissue time to recover, adapt and respond.