People - especially women over 35 who hold degrees in psychology - like to talk about spiritual growth and personal development, but are much less keen to talk about biological ageing.
Partly, this is because the violent changes to molecular and cellular structure over time invariably result in deterioration and death and no matter how priests, poets, and philosophers might like to dress it up, there's nothing fun about growing old and no one dies with dignity. In fact, death is the ultimate loss of dignity: a shipwreck into the nauseous, as Bataille so charmingly puts it.
The precise etiology of senescence is still largely undetermined and the process seems to be complex. Nevertheless, you can see it every time you look in the mirror, or, as here, by simply placing a series of photographs side by side showing the full ravages of time and decade after decade of fading youth and the failure of homeodynamics.