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“She is a teenage girl.”
She is a girl who is in her teens.
relating to the period of life between childhood and adulthood, typically ages 13–19
Teenage years are often marked by rapid emotional change.
The years when a person is a teen often involve quick emotional shifts.
Teenage pregnancy rates have declined in recent years.
The number of pregnancies among teens has gone down lately.
Used mainly as an adjective before a noun.
a person who is in their teenage years (rare as a noun)
The teenage was eager to join the club.
The teen was excited to become a member of the club.
More common noun form is "teenager".