The years of one's age having the termination -teen, beginning with thirteen and ending with nineteen; as, a girl in her teens.

One in two sexually active youth will contract an STD by at 25. Teens are at high risk for STD infection in part because of biological factors, such as an immature immune system. Behavioral factors that place teens at high risk include early age of first intercourse and multiple partners.

the time of life between the ages of 12 and 20

These are girls that pose or engage in sexual activity that either are or appear to be nineteen or under.

18 and 19 year old, barely legal young girls in the porn industry.