From Latin adolēscēns, the present participle of adolēscere 'to grow up' (ad- + alēscere 'to grow'), built on PIE *h₂el- 'to grow, to nourish.' This root also produced adult (past participle of the same verb), alumnus, aliment, proletariat, abolish, coalition, and even English old and world.
Of or relating to the period of development between childhood and adulthood; a young person in the process of growing from child to adult, typically between the ages of 13 and 19.
From Latin adolescens, present participle of adolescere (to grow up, to mature, to be nourished), a compound of ad- (toward) + alescere (to grow, an inchoative form), from alere (to nourish, to feed, to rear). The PIE root is *h2el- (to grow, to nourish, to rear). Latin alere also produced aliment (nourishment), alma mater (nourishing mother), alumni (those nourished by an institution), exalt (to raise up), and adult — the completed
English 'adolescent' and 'adult' are derived from the very same Latin verb, adolēscere — the adolescent is the present participle (one who IS growing up), while the adult is the past participle (one who HAS grown up). They form one of the most elegant grammatical doublets in the language: the same act of growing, frozen at two different stages of completion.