We can understand aging as being an accumulation of damage - of various types - to the biological machinery in and between our cells. This proceeds in an accelerating fashion as it passes various thresholds, and prompts evolved systems and feedback loops in our biology to flail, try to compensate, or fail in cascades. But in the very late stages of aging, for the oldest old, things stop being quite so easy to frame, and relationships between functional components - that hold throughout the rest of life - change dramatically. We see some of this change of phase in aging at work in the mortality plateau of flies; the risk of death per unit time in very old flies simply stops increasing, which poses interesting questions given that they are still alive and therefore presumably still accumulating biological damage. There are some signs that this late-life plateau of mortality rate might also exist in humans - but by no means enough data to be confident in that.
Here are a couple of papers from the Rejuvenation Research advance publication queue that also point towards the interesting nature of the late stages of aging. The first one is eye-opening, given everything we know about the effects of exercise at all other stages of life.
The relationship of physical activity (PA) and cardiac function in the oldest old remains unclear. The objective of this study was to evaluate the relationship between PA and cardiac structure and function, in the oldest old. Subjects were recruited from the Jerusalem Longitudinal Cohort Study that was initiated in 1990 and has followed an age homogeneous cohort of Jerusalem residents born in 1920-1921. ... After adjusting for sex, education, diabetes, ischemic heart disease, hypertension, dependence in activities of daily living, and body mass index (BMI), no significant associations were found between systolic or diastolic function, or left ventricular structure and PA. Gender-specific analyses yielded similar findings. Our study of the oldest old did not demonstrate an association between PA and cardiac structure or function.
In a variety of organisms, adulthood is divided into aging and late life, where aging is a period of exponentially increasing mortality rates and late life is a period of roughly plateaued mortality rates. In this study we used 57,600 Drosophila melanogaster from six replicate populations to examine the physiological transitions from aging to late life in four functional characters that decline during aging: desiccation resistance, starvation resistance, time spent in motion, and negative geotaxis. Time spent in motion and desiccation resistance declined less quickly in late life compared to their patterns of decline during aging. Negative geotaxis declined at a faster rate in late life compared to its rate of decline during aging.
These results yield two key findings: (1) Late-life physiology is distinct from the physiology of aging, in that there is not simply a continuation of the physiological trends which characterize aging; and (2) late life physiology is complex, in that physiological characters vary with respect to their stabilization, deceleration, or acceleration in the transition from aging to late life. These findings imply that a correct understanding of adulthood requires identifying and appropriately characterizing physiology during properly delimited late-life periods as well as aging periods.
I would like to see the advent of a world in which this sort of knowledge is a mere curio, as no-one ever experiences the final stages of degenerative aging, nor indeed does anyone even become what we'd now call old, aged, over the hill. Long lives, but no decline in vigor or health. That is a world in which the SENS research program has succeeded, and everyone has the opportunity to undergo periodic repair of metabolism, mitochondria, stem cells, and removal of harmful aggregates - no more remarkable than flu shots today.