also underage
1.
A person who is under age is legally too young to do something, for example to drink alcohol, have sex, or vote.
Underage youths can obtain alcohol from their older friends.
...girls who have babies when they are under age.
ADJ
2.
Under age activities such as drinking or smoking are carried out by people who are legally too young to do them.
...his efforts to stop under age drinking and drug abuse.
ADJ : ADJ n