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.
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.
