Does being fit or muscular help to stay in better shape as people age? I have to ask as intuition would say yes but there don't seem to be serious studies on the subject. Also, some athletes are said to end up with a bit of wear and tear early in life. There could be a point beyond which exercise and build could be detrimental.
There are several aspects to aging, and depending on what you do will affect how you age. It's not as simple as "just be fit and you'll stay healthier". Apparently there is a book on the subject that will provide a much rounder picture than I can provide here.
- Sarcopenia is the sad fact that you will lose muscle as you age. However, depending on the exercise you do, you can slow the rate of loss. If you do nothing (remain sedentary), you will lose 15% of your muscle mass per decade after you turn 30. Dr. Kilgore mentioned in an article that resistance training can reduce the muscle loss from sarcopenia to only 5% per decade.
- Osteoporosis is another age related ailment that resistance training can either prevent or at least slow (see the Dr. Kilgore article above). Essentially the bones become less dense and more porous. Resistance training puts additional stress on the bones which forces the adaptation for your body to deposit more calcium on them.
- Cardiovascular health is influenced by conditioning (not resistance training). Proper conditioning will build your heart and lungs to support the other activities you are involved in. If you keep doing them as you age, your cardiovascular system will be much healthier.
- No special name, but your tendons and connective tissue tends to get more brittle with age. Exercise and stretching help keep them from being injured as easily. Of course, exercise can also cause them to be injured in the first place. Best course of action is a little more at a time.
Outside of these major quality of life and health concerns, proper diet and hydration affects your skin tone, fat content, blood sugar levels, etc. The sad truth is that there are no guarantees that will give you a high quality of life until you die. However, there are things you can do to increase your odds.
Depends on what you mean by "aging better". Aging has a lot of different aspects, for example the loss of health (e.g. increased cardiovascular risk), loss of physical fitness, and deterioration of physical appearance.
There is no question at all that exercise prevents age-related health problems, and there are countless studies on that subject. Here's a study showing that exercise reduces cardiovascular risk. The same is true for physical fitness: Obviously exercise helps you stay fit as you age, since that's the main purpose of exercise in the first place.
Perhaps you are more interested in the age-related effects on appearance, i.e. does exercise help us look young as we age? This is a more complex topic, since appearance is fairly hard to measure.
One part of it is wrinkles, and I would argue that staying muscular is definitely a great way to prevent body wrinkles, by keeping the skin tight. In addition, skin hydration is a big factor in preventing wrinkles and exercise keeps you and your skin hydrated by strengthening your heart and improving your blood circulation.
Another part is how "healthy" you appear in terms of your skin tone, which is also affected by cardiovascular strength (blood circulation) and as such definitely associated with exercise.
Yet other parts are your stress-resistance, energy levels and emotional stability (sort of your "behavioral appearance"), all of which may decrease with age, and all of which are significantly improved with physical exercise.
So there's no doubt that exercise slows almost every aspect of aging. Is exercise also associated with increased risk of injury? Yes, but injury is something you can control and prevent to some extent if you're smart, while the creeping effects of age-related decline are not (unless you exercise).
The "wear and tear" kind of injury is mostly associated with excessive exercise, such as running the length of one or two marathons every week of your life. It's entirely preventable by moving from low- or moderate-intensity exercise to high-intensity exercise of shorter duration. But that's a different topic.
This study/article from the Stanford School of Medicine looks at older runners over almost twenty years. It shows that:
Elderly runners have fewer disabilities, a longer span of active life and are half as likely as aging nonrunners to die early deaths ...
Here's a paragraph that explains the methodology of the study:
Fries’ team began tracking 538 runners over age 50, comparing them to a similar group of nonrunners. The subjects, now in their 70s and 80s, have answered yearly questionnaires about their ability to perform everyday activities such as walking, dressing and grooming, getting out of a chair and gripping objects. The researchers have used national death records to learn which participants died, and why. Nineteen years into the study, 34 percent of the nonrunners had died, compared to only 15 percent of the runners.
These two paragraphs describe the eventual onset of physical disabilities:
On average both groups in the study became more disabled after 21 years of aging, but for runners the onset of disability started later.
“Runners’ initial disability was 16 years later than nonrunners,’” Fries said. “By and large, the runners have stayed healthy.”