Ooooh, interesting topic!
OK, a few random thoughts:
Morbidity vs mortality
It's important to distinguish between these two. I have no desire to live past 80 if I have very poor quality of life beyond this point. However, I try to lead a healthy life so that I compress morbidity as much as possible. Dying in a surfing accident at 105 years old would be fine by me! (note to self: learn to surf)
"Natural" life expectancy
An interesting question... why do women live beyond reproductive age? For much of our evolutionary history this was probably a rarity so it didn't confer an evolutionary advantage, but having grandmothers around is likely to increase the survival of their grandchildren. Clearly there is some sort of limit to how long people can live, but we may not in fact be all that close to it yet. Average life expectancy is still rising.
Sport and health
People often hold up athletes as paragons of health, but they aren't - they are paragons of performance. Ignoring the issue of performance enhancing drugs, there comes a point where there is a trade-off between performance and health. Yes, exercise is good for you but you can have too much of a good thing.
It is also important to remember that all sports are an abstraction of proper human movement. We are all inherently capable of a huge diversity of movement, and each sport taps into just a small subsection of that. This is the source of a lot of injuries, from over-use, muscle imbalances, over- or under-joint mobility etc.
I think exercise is hugely important for the health of people of all sizes, but not all exercise is created equal and doing too much or operating in too narrow a field can be detrimental.
Natural weight
I'm going to stoke a little controversy and say that it's not natural to be very overweight... but that's because our bodies are responding to a very unnatural stimulus. Processed foods are designed to be over-consumed by companies seeking bigger profits. They are made hyper-palatable and moreish as this is good business. To get fat on real food (meat, fish, vegetables, tubers, fruit, nuts, good fats etc.) is not easy as our bodies are very good at telling us when we've had enough of these foods. Junk food short-circuits these hormonal mechanisms so we don't feel full. They break us, and that's not natural.
Weight and health
There are many diseases that are associated with being overweight - heart disease, diabetes etc. However, these aren't necessarily caused by being overweight per se. I'd argue that both being overweight and having these diseases is symptomatic of the kind of poor diet I outline above. This leaves room for being overweight and being healthy. Eat right (real foods, not packaged stuff with "Low fat" or "Healthy" written on it) and move around. You aren't likely to be huge if you do this, but you can certainly be overweight and healthy.
Sociological issues
Yup, I believe these play a big part. Poverty is an issue as the poor fall victim to the poor quality food pushed by the food industry, partly through means (i.e. it's cheap), partly through culture (e.g. peer pressure) and partly through lack of education. As pointed out above, stressors such as financial uncertainty, poor housing and low relative social standing also have a negative effect on health. In the US in particular, a lack of access to healthcare is a big (and frankly appalling) issue.
I agree with Lancila that emotional health is important too. In fact, I'd argue that both physical and emotional health are intertwined far more than most people would think.
12 years