Definition of adolescent:

displaying or suggesting a lack of maturity

Synonyms of adolescent:

jejune, juvenile, puerile, teenage, youth, youthful, minor, young, immature

Opposite/Antonyms of adolescent:

adult, elderly, matured, grown-up, old, gray, big, older, infant

Sentence/Example of adolescent:

The trouble was in his conditioning, started when he was an adolescent.