You must be for abstinence for young people, then, as well? That would be a consistent stance with what you just said. There are numerous 12-year-olds having intercourse who are not being raped.
Typically, kids under the age of 13 who find themselves with STI's and/or pregnancy are discovered to have been victims of sexual assault. There are outliers, of course, in any situation. But the majority of cases of underage pregnancy and STI's are resulting from rape at the hands of a family member or trusted adult.
Pregnancy of underage girls are at a higher rate of infant and maternal mortality than women over 18-19. Bodies have not completely developed yet, in spite of the ability to gestate somewhat. It's like the difference between someone who's pre-frontal cortex in their mid-20s on average will act less impulsively than a teenager, hence insurance rates in the U.S. are higher for teen drivers than adult drivers over 25.
Underage girls who are pregnant will suffer more than adult women in pregnancy, labor, and childbirth. But rather than take sex away as any option for teens (typically teens begin consensual sex with peers around 15-17), I say treat it like learning to drive. It's a huge responsibility to drive a car and to learn safe driving and to know what to be aware of. Don't text and drive. Don't drink and drive. Follow the rules of the road for your safety and for everyone elses safety.
With sex, be aware of the risks. Don't try to have sex with somebody who isn't consenting (learn what consent is first and foremost). Use protection. Be aware of who you are having sex with in case their is greater risk for STI's if they are not disclosing sexual activity with others or are not disclosing their sexual health status.
When there is comprehensive sex education and greater support for teens, as well as greater support for adult men and women, to utilize accessible contraceptives, the number of unwanted pregnancies go down significantly. Less unwanted pregnancies. Less elective abortions.