The human body needs to be taken care of in a variety of ways for best performance over the long term. Exercise, keep the weight off, try to avoid stabbing yourself. It's a considerable disadvantage that in our formative years that sort of maintenance just happens as a natural consequence of being a child - so the additional work that has to go into maintaining health as an adult comes as an unexpected chore.
So: many of us get successful, then get fat, and then suffer age-related conditions more frequently and sooner, and then on average die younger. This isn't rocket science - most people know what they are doing to themselves, even if they aren't up to speed on the details of the biochemistry involved. But the siren song of life in a time of wealth and plenty lures you in. Maybe medical science will save you from yourself ... but I wouldn't count on it.
So maintain yourself. You stand on the verge of a golden age in biotechnology, one that will offer unlimited healthy, youthful lifespans to those who claw their way over the threshold. Slacking on your health is turning your back on that future, it is making it harder for you to live long enough to benefit from rejuvenation biotechnologies that can be clearly envisaged today.
An item to reinforce the point on maintenance:
Exercise can counteract muscle breakdown, increase strength and reduce inflammation caused by aging and heart failure. The benefits for heart failure patients are similar to those for anyone who exercises: there's less muscle-wasting, and their bodies become conditioned to handle more exercise. Age of the patients didn't matter, either, researchers found. ... These findings offer a possible treatment to the muscle breakdown and wasting associated with heart failure and suggest that exercise is therapeutic even in elderly heart failure patients. The findings also suggest an avenue for drug development to slow muscle breakdown in heart failure patients. ... Exercise switches off the muscle-wasting pathways and switches on pathways involved in muscle growth, counteracting muscle loss and exercise intolerance in heart failure patients.
A secondary point is that you have to be proactive; maintenance doesn't just happen. This is likely why there are correlations between age-related disease and depression, or other measures of mental state. The influence of exercise and other forms of maintenance are strong enough to cause many other secondary correlations - so to my eyes it's not the purpose, it's what you do with it.
Our study showed that people who reported greater purpose in life exhibited better cognition than those with less purpose in life even as plaques and tangles accumulated in their brains ... These findings suggest that purpose in life protects against the harmful effects of plaques and tangles on memory and other thinking abilities. This is encouraging and suggests that engaging in meaningful and purposeful activities promotes cognitive health in old age.
People who had depressive symptoms in both midlife and late life were much more likely to develop vascular dementia, while those who had depressive symptoms in late life only were more likely to develop Alzheimer's disease.