as you get older, your metabolism slows down, and your body does not function as well as it used to when you were younger. But being 25 is still young! Manly to stay fit, is to exercise and yes you can be more fit than a 16 year old because since your body is not changing that much anymore, you do not have to worry about where proteins and vitamins go. Before your body would absorb them because of the hormonal and other changes taking place, but now you can use them to gain muscle, lose weight and stay fit Just remember to stay in shape and run/work out at least 3 times a week. And if no physical damage happens to you (hopefully it wont) you will stay healthy and fit for as long as you live unless you start eating junk food, and not exercising, and being a couch potato..aha