Diminishing returns. If you address some of the top causes of death, you can significantly increase lifespans. If you address some of the smaller causes, you can somewhat increase lifespans. However, in the limit you need a solution that doesn't just solve individual problems one by one. Biologically, "fixing" telomeres would probably address many of the nonspecific "died of old age" cases that we don't fully understand yet, though I doubt that alone would fix the general problem.
More importantly, we need people to care, and for some insane reason people don't. How do we manage to not treat this as pretty much the most important unsolved problem in humanity? (Mostly, I suspect, due to a combination of perceived futility and ingrained cultural problems.)
There's a great TED talk a few years ago on this. We have the technnology and know-how to significantly increase life expectancy and to do research to make it even better, but why don't we? Curing cancer and Alzheimer's is not the same thing. Why don't we explicitly research life extension? The speaker brought up several philosophical, political and economic reasons why society has chosen to ignore the problem. There are a lot of unforeseen implications to a world where everyone lives past 150.
I'm not sure why that makes any difference. The level of medical care I can get (and have gotten) as a middle class American blows away what these people can get now. The disparity is so huge that adding 100 years to my life barely makes a dent in the disparity.
More importantly, we need people to care, and for some insane reason people don't. How do we manage to not treat this as pretty much the most important unsolved problem in humanity? (Mostly, I suspect, due to a combination of perceived futility and ingrained cultural problems.)