I'm really aware of aging issues. Life itself is a degenerative process. But you still have considerable control over the quality of life as you age. When you understand the innate changes in your body over time, you are better able to craft a lifestyle to maintain optimal health and reduce the chances of catastrophic problems. 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.