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
It is a truth universally acknowledged that with senescence we grow both wiser and more forgetful. I used to know why this happened but I've forgotten the answer.