Well, teenagers bodies haven't really fully developed. So even if there using birth control unplanned pregnancy can happen. When a girl is in her teen's taking birth control, the birth control basically doesn't have any effects upon the user because, she is young and nothing is really developed yet making in easier for a young teen to become pregnant.
If a woman in her 30's is taking birth control her chance of becoming pregnant because, her body is fully developed. So the birth control works properly.
