The main reason would be that some forms of birth control may be less effective for teenage girls because their bodies haven't fully matured and settled into a regular menstrual cycle. In addition teenage girls are more fertile than women in their thirties and have a higher chance of conceiving even while taking birth control.
A teenage girl who is using birth control be more at risk for becoming pregnant than a woman in her thirties because a teenage girl is very fertile at this point than women in thirties. Also, their health are at much better state.
