adolescent (n.) Look up adolescent at Dictionary.com
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- (3) "to grow, nourish" (see old). 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.
adolescent (adj.) Look up adolescent at Dictionary.com
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.)).