adolescent (n.)
mid-15c., "youth, young man," from Middle French adolescent (15c.) or directly from Latin adolescentem (nominative adolescens) "growing, near maturity, youthful," present participle of adolescere "grow up, come to maturity, ripen," from ad- "to" (see ad-) + alescere "be nourished," hence, "increase, grow up," inchoative of alere "to nourish" (see old). Adolesce was a back-formed verb used early 20c. by H.G. Wells, G.B. Shaw, Louis MacNeice, but it seems not to have taken.
adolescent (adj.)
1785, from Latin adolescentem (nominative adolescens) "growing, near maturity, youthful," present participle of adolescere "grow up, come to maturity, ripen" (see adolescent (n.)).