Source : Webster's Revised Unabridged Dictionary (1913)

Senescence \Se*nes"cence\, n. [See {Senescent}.]
   The state of growing old; decay by time.

Source : WordNet®

senescence
     n 1: the organic process of growing older and showing the effects
          of increasing age [syn: {aging}, {ageing}]
     2: the property characteristic of old age [syn: {agedness}]