teenager UK /ˈtiːnˌeɪ̯d͡ʒə/US /ˈtinˌeɪdʒɚ/ noun1 •a young person older than 12 and younger than 20, no longer a child and not yet an adultMost teenagers will undergo lots of changes before reaching adulthood.Marco's main target market are late teenagers. formsteenagers