"condition of growing old, process of decaying by time," 1690s; see senescent + -ence. Related: Scenescency (1660s in the same sense).
"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.