At any age, it's difficult to get in shape if you are way out of shape. But when you are young, I maintain it's difficult to get way out of shape or way overweight.
You're growing, and your body needs a tremendous amount of calories just to maintain the growth. You have an advantage in that if you also do something athletic that burns calories, you can get amazing bodies at little physical expense (meaning in terms of workout regime and dieting, which you won't have to do).
On the other hand, if you are 50 y.o., you are certainly not growing any more. The calorie balance equation is tipped towards the "not needing many calories to exist" part of the equation. So you have to be careful.
That said, I agree with several posters above. I have stayed in good shape because I have made it a lifelong pursuit. It's a life style issue. I consider working out (either running, or weight training) as much a part of my life as brushing my teeth (as someone above put it as well). Not only that, I love to do it. It is not onerous.
It's as much a health "thing" for me as it is a vanity/gay thing. And, once you are in shape, and older, the interesting thing is that you do indeed NOT look your age, because the typical older guy is so out of shape...people do not seem to realize that so much they attribute to age....pot bellies, love handles, double chins...is really under their own control.