Life science researchers are very interested in establishing biomarkers that are fairly reliable measures of physical age, mortality risk, and remaining life expectancy. Without quantifiable biomarkers, how do you determine whether a potential rejuvenation technology is actually working? The only other way is to wait and see how long it takes your subjects to die: impractical in humans and costly in laboratory mice. The field of longevity science is very much in need of ways to rapidly determine the effectiveness of a given therapy - because otherwise you're stuck testing for three years in mice to even qualify an approach as potentially useful. Which few will be, of course, but it'll take those three years and a pile of money to find out in each case.
Related to this issue, I noticed an open access paper that explores how well the rate of change in common measures of health can predict mortality. As it turns out, quite well - better than considering snapshots in time. This makes good sense: aging is exactly the accumulation of damage and failure of systems. People who are failing faster will see their health measures change more rapidly in the crucial years between age 40 and age 60, when biochemical damage starts to seriously impact metabolism and health.
It is well known from epidemiology that values of indices describing physiological state in a given age may influence human morbidity and mortality risks. Studies of connection between aging and life span suggest a possibility that dynamic properties of age trajectories of the physiological indices could also be important contributors to morbidity and mortality risks. In this paper we use data on longitudinal changes in body mass index, diastolic blood pressure, pulse pressure, pulse rate, blood glucose, hematocrit, and serum cholesterol in the Framingham Heart Study participants, to investigate this possibility in depth.
We found that some of the variables describing individual dynamics of the age-associated changes in physiological indices influence human longevity and exceptional health more substantially than the variables describing physiological state. ... We showed that the rate of changes in physiological state at the age interval between 40 and 60 years may serve as a good predictor of morbidity and mortality risks later in life. For nonmonotonically changing indices, the rates of decline after reaching the maximum, the maximal values, and the age at the maximum are important predictors of morbidity and mortality risks.
It's worth reading the whole thing (or skipping to the last quarter at least, if statistical discussion makes you queasy) for a sense of which changing values were shown to be more important than others. The take away here is that actively maintained stasis is important: the longer you can keep your measurable health parameters looking like they did when you were 40, the better your chances. Making a serious go at this will of course require a sane diet and good exercise regimen at the very least - and this is likely why those people whose measurements changed to a lesser degree were better off.
No presently available medical technology does as much good for the healthy as exercise and calorie restriction. Which is a state of affairs we'd all very much like to change - longevity medicine that can provide additional decades of healthy life is very possible, very plausible, and not too many decades away if the funding is forthcoming for strategies like SENS.