According to the National Survey of Sexual Health and Behavior—and contra the impression presented by Hollywood—the vast majority of teenagers are not having sex:
Many surveys of adolescent sexual behavior create an impression that adolescents are becoming sexually active at younger ages, and that most teens are sexually active,” Fortenberry said. “Our data show that partnered sexual behaviors are important but by no means pervasive aspects of adolescents’ lives. In fact, many contemporary adolescents are being responsible by abstaining or by using condoms when having sex.”
[. . .]
At any given point in time, most U.S. adolescents are not engaging in partnered sexual behavior. While 40 percent of 17 year-old males reported vaginal intercourse in the past year, only 27 percent reported the same in the past 90 days.