I believe that's a fairly well-understood effect: the physical characteristics that we associate with 'age' are literally the visual symptoms of the body gradua

I believe that's a fairly well-understood effect: the physical characteristics that we associate with 'age' are literally the visual symptoms of the body gradually falling into disrepair. This study seems to go a bit deeper, testing whether this heuristic assessment of 'age' from the states of visible systems is a useful proxy for the 'age' of non-visible ones. If there were no relationship, one would expect that apparent age would have no identifiable correlation with longevity.