Underage (adjective)

Not of legal age; not legally old enough to do something, especially to drink alcohol or vote.

Origin:

Under + age.

Examples:

1. The bouncer at the bar refused to let him in because he was underage.
2. She was caught buying cigarettes underage at the convenience store.
3. His parents were worried he would get into trouble because he was underage and hanging out with older kids.
4. The voting age in this country is 18, so anyone underage is not able to vote.
5. He was caught with alcohol at a party and charged with underage drinking.