"condition of growing old, process of decaying by time," 1690s; see senescent + -ence. Related: Scenescency (1660s in the same sense).
Entries linking to senescence
senescent adj.
"growing old, aging," 1650s, from Latin senescentem (nominative scenescens), present participle of senescere "to grow old," from senex "old" (from PIE root *sen- "old").
Blount's "Glossographia" (1656) also has a verb senesce "to wax old, to grow in age, to begin to decay or wear away," from Latin, and OED has quotes for it from Stevenson, Shaw, and J.D. Salinger.