It is common to lose muscle mass, although this is largely the result of less activity. Range of motion becomes limited as muscles shorten, tendons and ligaments become less elastic. Some degree of arthritis might affect the bones of the joints. Muscles remain responsive to exercise and stretching, but require regular use to prevent significant, functional weakening. The skin becomes less elastic and more thin, bringing with it increased risk of bruising, cuts, and skin breakdown from pressure.
Bones become more brittle.
Osteoporosis is common in elderly people, particularly women. Being at higher risk for breaking bones as you age is not just because you become less stable and fall more, but because your bones have lost calcium and therefore break more easily. As you age, your senses lose sensitivity, reflexes slow down, coordination is reduced. Energy levels get lower, and you need less sleep. These facts of life are not inevitably limiting.