That definition changes as you get older. I kind of sigh for those old days, not that I ever met the criteria. At 48, I think of a "great body" as one that is reasonable in weight and that gets me from point A to point B with a minimum amount of pain and suffering. And, actually, this is an achievable goal. So how do you get a middle-aged "great body"? Well, you can borrow from the kids--they know to be buff you have to exercise a lot. And you have to tone your muscles with weight lifting.
The nice thing is that while you might be doing the same exercises that the youngsters are doing, but you actually have a chance of achieving your goal whereas genetics may limit their ability to attain their goals! It is not easy, but it is possible to fit exercise in on a daily basis. It has to become a priority, though, in order to do so. That means you have to map out your day with the thought to an hour a day for moving your body.
An hour a day can be achieved all at once or it can be achieved in small amounts. Park at the back of your parking lot at work and find out how much time it takes to walk from your car to your station. That time counts as exercise. If it takes five minutes and you drive yourself to lunch every day (even a park where you can eat a homemade sandwich) then you have 20 minutes of exercise right there.
You can work your way up to jogging and even running a marathon if you like. Or you can keep it at a walk, but there's really no excuse for not doing something. You know the other thing the kids do--they watch what they eat. Well, maybe they drink too many caffeine-loaded energy drinks, but the ones who are serious about how their bodies look are likely to eat somewhere in the ballpark of "right."
This means nutritious food. Fruits and veggies. Yogurt. Actually there are ways of eating the right foods even if you are not eager about consuming plants. Smoothies with frozen berries, an orange, and an apple ground up in the blender (and a little artificial sweetener) are very tasty. Stir fried (in a very small amount of grease) veggies with a teriyaki-based sauce are also good. You can have the great body--of your definition. It takes work and commitment, but it can be achieved and you can do it.