underage

adj. not being old enough; under the legal age, juvenile, minor

In many countries such as India, the UK, Australia, Canada and New Zealand a minor is presently defined as a person under the age of 18. In the USA, the term "minor" is not entirely synonymous with the term as a minor may be tried for a crime (and punished) as a juvenile or an adult. Also, in the USA, the age considered to be a minor varies from State to State, usually being 18 or 21.
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Adjective
1. not of legal age; "minor children"
(synonym) minor, nonaged
(classification) law, jurisprudence
2. dependent by virtue of youth
(similar) dependent

    dungaree

Synonyms and related words:
budding, callow, defalcation, deficiency, deficit, dewy, green, growing, immature, impubic, inadequacy, inexperienced, ingenuous, innocent, insufficiency, intact, juicy, lack, minor, naive, new-fledged, raw, ripening, sappy, scantiness, shortage, tender, unadult, undeveloped, unfledged, unformed, unlicked, unmellowed, unripe, unseasoned, vernal, virginal