The main reasons for this is that once we reach the age of 30, the muscles in our body begin to shrink because they are not being used correctly. Also our bones become much less dense over time. In some cases this may actually lead to a person suffering from osteoporosis. Unfortunately as our muscles begin to shrink so our metabolic rate also begins to slow down as well. its hard to keep the muscles and a flat body you had, However there are things a person can do when they reach the age of 40 which can help to prevent them losing muscle mass and keep their metabolic rate working at a good level. You have to follow the right kind of diet and use a good exercies.