No matter who you are, your dietary and exercise needs change as you grow from a child to an adult and then into old age. There is no single diet or exercise routine for every age group. Your age is one factor that determines what you should eat and how much exercise you do.
But more importantly, what you eat and how you exercise while you are young will affect your health for the rest of your life. You cannot go back in time, but you can prepare your body for the future.