Answer:
Senescence.
Explanation:
As the exercise suggests, what Spencer noticed in himself (his aging) was due to senescence. This concept represents the biological aging, the natural deterioration of our abilities, be it physical or otherwise. We have felt this, in some way, if for example we've noticed something we couldn't do when we were younger (let's say run for 30 minutes). This is the opposite: as we grow progresively older, we start to change, to deteriorate, to progress with the natural cycle of life.
