From Longman Dictionary of Contemporary Englishunderageun‧der‧age /ˌʌndərˈeɪdʒ◂/ adjective PGCtoo young to legally buy alcohol, drive a car, vote etc underage drinking
Examples from the Corpus
underage• They also express concern about the addiction and underage gambling they believe will be the inevitable partners of online gambling.• Tilden died broke, and shunned by many because of his conviction for homosexuality with underage partners. 