Humans are not laboratory mice, whose lives can be styled to best eliminate confounding factors from studies that aim to demonstrate a nice, neat correlation between cause A and result B. So when you study humans for decades, living their complicated and individualistic lives, you get all the confounding factors you can handle and then some. Take studies of fat, for example, and the strong correlation between extra body fat carried over the years, age-related illness, and a shorter life:

American researchers observed more than 17,000 female nurses with an average age of 50 in the U.S. All of the women were healthy when the study began in 1976. Researchers then monitored the women's weight, along with other health changes, every two years until 2000. For every one-point increase in their Body Mass Index, women had a 12 percent lower chance of surviving to age 70 in good health when compared to thin women. Researchers defined "healthy survival" as not only being free of chronic disease, but having enough mental and physical ability to perform daily tasks like grocery shopping, vacuuming or walking up a flight of stairs. ... For every 1 kilogram (2.2 pounds) gained since age 18, women's odds of surviving past 70 dropped 5 percent, researchers found. Women who were already overweight at age 18 and then gained more than 10 kilograms (22 kilograms) later in life only had about a 20 percent chance of surviving to age 70 in good health. The most commonly reported diseases were cancer, heart disease, and diabetes.

Fairly straightforward, you might think. But wait: excess body far is also inversely correlated with wealth:

The point here is that sickness, poverty, and obesity are spun together in a dense web of reciprocal causality. Anyone who's fat is more likely to be poor and sick. Anyone who's poor is more likely to be fat and sick. And anyone who's sick is more likely to be poor and fat.

Interestingly, wealth isn't all that correlated with longevity in comparison to body fat:

The billionaires lived 3.5 years longer than average American males. The results would be even more dramatic if we took into account average life expectancies from around the world, since the billionaires on our list are of all different nationalities. According to a 1999 study in the British Medical Journal, higher income is, in fact, "casually associated with greater longevity." But when it comes to living longer, billionaires may not be that much better off than mere millionaires. "While an extra dollar of income is protective," the study reads, "the amount of protective effect tails off as total income rises."

But we can't forget that measures of intelligence correlate with wealth, health, and keeping slim:

Some studies contend that rich live longer because of intellectual Darwinism. "Social status," Seligman writes, "correlates strongly and positively with IQ and other measures of intelligence, and intelligence correlates strongly with health literacy - the ability to understand and follow a prescription for disease prevention and treatment." This theory is not without evidence: Seligman cites a 2003 study by psychologist Ian Deary of the University of Edinburgh that found mortality rates to be 17% higher for each 15-point falloff in IQ.

At this point the layman shrugs his shoulders and points out that people who are better off tend to have become better off by taking an interest in becoming better off - an attitude which doesn't stop at making money. For the vast majority, those people lucky enough to have avoided snake eyes in the genetic lottery, being healthy is both a choice and a vocation. Some people work at it, and some people don't. Those who take an interest in their health and do the work required tend to reap the rewards.

From my point of view, I think that the evidence is very strong for extra body fat - visceral fat, to be specific - as a major contributory cause of accelerated age-related degeneration. By this I mean a direct and biochemical cause, not merely a correlation; people aren't degenerating more rapidly as a direct result of a lack of dollars or IQ points, but changes to the operation of human biochemistry brought about by the presence of larger visceral fat deposits are very damaging over the long term. Aging is absolutely a matter of how much damage your biochemistry has sustained, and the present consensus is that fat-induced inflammation has a lot to do with accumulated damage:

Chronic inflammation spurred by an immune system run amok appears to play a role in medical evils from arthritis to Alzheimer's, diabetes to heart disease. There's no grand proof of this "theory of everything." But doctors say it's compelling enough that we should act as if it were true - which means eating an "anti-inflammatory diet," getting lots of physical activity, and losing the dangerous, internal belly fat that pumps out the chemicals that drive inflammation.

We have previously demonstrated that surgical removal of visceral fat (VF) in rats improves insulin action, thus, we set out to determine if VF removal affects longevity. We prospectively studied lifespan in 3 groups of rats: ad libitum fed (AL), 40% caloric restriction (CR) and a group of ad libitum fed rats with selective removal of VF at 5 months of age (VF-). We demonstrate that compared to AL, VF- rats had a significant increase in mean and maximum lifespan and significant reduction in the incidence of severe renal disease.

You might also take a look at this mouse study for a fairly direct demonstration that fat shortens life:

It's all something to think about, especially given that being overweight is a choice. An easy, seductive choice perhaps, but a choice nonetheless. You could do something about it, and hence improve your chances of living to see an era in which longevity medicine restores the elderly to health and vitality.