Contraception is most commonly used to balance hormone levels of under-age girls and regulate their periods. But, it should be available anyways because teens aren't going to choose not to have sex because of a lack of birth control. Making it illegal would just increase the teen pregnancy and STI rates.
Doctors should not prescribe contraception to under-age girls because those under-age girls should not be in circumstances that merit the prescription of contraception. If a prescription is warranted, perhaps it would be for the best if the girl went elsewhere for a while.
Also, it could really mess up their hormones.