The word "juvenile" can be used as an adjective or as a noun when describing a person who is a child or a teenager–anyone who is under the age of 18. This word is commonly used in formal settings such as a hospital or a court of law:
The police arrested two female juveniles who were caught breaking into a store.
They were placed in juvenile detention.
Children under the age of eighteen are usually tried in a juvenile court.
The doctor operated on a juvenile patient.
Juvenile diabetes is a serious problem in the United States.
Sometimes the word "juvenile" is used to describe childish behavior for children and adults:
That’s so juvenile!
There’s way too much inappropriate and juvenile behavior in that office.
Our class has a lot of juvenile delinquents in it.
Her juvenile attitude is what gets her into trouble.
He won’t go to jail for this because he’s a juvenile.