Throughout your life you will see many changes. The most dramatic of these usually occur to the body. From infancy to old age, your body will go through many stages. Apparently every 7 years every cell in your body has been re-newed. So effectively the body you have now is a different one to the one you had seven years ago!
How your body copes and develops with these changes depends on how it is treated. We all know the benefits of exercise, good diet and abstaining from drugs (tobacco and alcohol included). The issue is what we do with this information. I do not drink alcoholic and have never taken drugs (apart form the occasional paracetamol. I eat what I believe to be a balanced and healthy diet (with occasional treats); I exercise daily (yoga) and increase my activity levels when I have time and the inclination. I never force myself to do activity as this creates a negative association. I often reward myself for being active by treating myself, although I often find the reward is the activity itself, even if the benefits are not seen until later.