late 15c., from M.Fr. adolescent, from L. adolescentem (nom. adolescens), prp. of adolescere "grow up," from ad- "to" + alescere "be nourished," hence, "increase, grow up," inchoative of alere "to nourish" (see old). The adj. is first attested in 1785. 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.