Underage means younger than the law allows for something. Underage drinking, an underage driver, serve somebody underage.
Which age counts depends entirely on the activity and the country, so somebody can be old enough to marry and underage to vote in the same place. Selling alcohol to an underage customer is the shop's offence rather than the customer's.
1adjectiveunderageYounger than the law allows for something.
“Underage workers were found sewing clothes in the factory.”
underage is a legal state.
Under plus age. The compound is recorded from the sixteenth century in legal use.
From under- + age.
Etymology from Wiktionary, CC BY-SA