adolescent (n.)

mid-15c., "youth, young person, one who is growing up," from Middle French adolescent (15c.) or directly from Latin adolescentem/adulescentem (nominative adolescens/adulescens) "young man or woman, a youth," noun use of an adjective meaning "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," from a suffixed form of PIE root *al- (2) "to grow, nourish." Adolesce was a back-formed verb used early 20c. (OED quotes H.G. Wells, G.B. Shaw, Louis MacNeice), but it seems not to have taken.

1785, "growing toward maturity," from Latin adolescentem/adulescentem (nominative adolescens/adulescens) "growing, near maturity, youthful," present participle of adolescere "grow up, come to maturity, ripen" (see adolescent (n.)).