Type 2 diabetes is the poster child for an avoidable age-related condition: barring the worst of genetic bad luck, calorie restricted, well exercised people will not suffer from type 2 diabetes. But this is, undeniably, an age-related illness. Becoming ever more obese and sedentary will hasten the onset of diabetes into ever earlier years of life, but older obese and sedentary people are still far more likely to suffer type 2 diabetes than are equally overweight and sedentary younger counterparts. So while failing to take care of your health at any age is just another form of self-harm, there are other, less avoidable processes taking place at the level of cells and organs that make older people more vulnerable.
Here is an open access paper that reviews what researchers presently know of the decline of insulin-producing beta cells in the pancreas - which turns out to be not enough, as is still true of so much of our biochemistry. There are changes, cataloged and identified, but the chains of causation for those changes are poorly understood at best.
An increased incidence of diabetes is observed with age, and there are many possibly reasons for this. One of these is that the beta cell has reduced proliferative capacity and in diabetic individuals this is further confounded by higher rates of beta cell apoptosis. The currently known underlying mechanisms behind the reduction in beta cell proliferation observed with age include reduced expression of cell cycle activators, increased expression of cell cycle inhibitors, reduced pdx1 expression, and increased amylin aggregation. Studying aging in the non-diabetic rodent and human models is currently a developing field; therefore very few broad conclusions can be drawn. Further study in these areas is important as they could indicate targets for preventing or slowing the progression of diabetes with age.
I look on this as a good illustration of why the detailed, tissue by tissue, understand everything approach to repairing aging is doomed to take a very long time indeed. This is but one population of vital cells in one organ, one of the most studied forms of cell in past decades, and the research community remains far from a complete understanding as to how and why they fail with age.
Better strategies to deal with aging exist - such as SENS - and need to gain wider support and adoption. SENS-like approaches work around the challenge posed by the sheer complexity of human biochemistry by focusing on the known common mechanisms of aging, the root causes from which there is good reason to believe all other changes descend. Repairing these root causes is the fast path to the first generation of rejuvenation biotechnology, and that, in my eyes, is the only real shot at building viable interventions in the aging process that will arrive in time to help us.