Though adolescent fertility rates have declined in the United States as well as nearly all western countries, the US rate is notably higher than most other western countries. These changes are not due to how early and how frequently teenagers have sex. Instead studies show contraception use and sex education have a significantly greater impact.
Unlike some other countries, the United States currently requires women to have an exam and receive a prescription before going to a pharmacy to get birth control pills. In addition, conservative politicians and companies have pushed to not require coverage of contraception, with some arguing it will entice sexual activity. Amy Schumer has parodied this process here as well as the following clip.
As to sex education, it is inconsistently offered and when available parents are able to opt out.