This is a fancy word for teenager. After puberty has begun but before adulthood is known as the adolescent phase.
This word refers to all things related to teens — teenagers are adolescent, and the adolescent stage is after puberty and before adulthood. Since adults don't have a high opinion of teenagers, this word is also an insult. Movies full of fart jokes and immature shenanigans are called adolescent. If one adult says to another "You're acting very adolescent" that means "You're acting childish." Adolescent means something close to juvenile, another word that refers to young people and childish people.
