Senescence

senescence \si-ˈne-sən(t)s\ noun: 1. the state of being old; the process of becoming old, 2. the growth phase in a plant or plant part (as a leaf) from full maturity to death

Etymology: senescent, from Latin senescent-, senescens, present participle of senescere to grow old, from sen-, senex old

Compare this word to April 17’s juvenescence.