This isn't really factual. Adolescents' brains are mostly developed with the prefrontal cortex being the last part of the brain to fully develop. As a result adolescents are more likely (but do not always) make more impulsive decisions.
With that said though, the idea that they 'cannot have a brain developed enough to monitor and make changes to their behavior" is ridiculous. By the time someone reaches 15, they have already undergone most of the development related to metacognition and the ability to think in abstraction and are more cognitively similar to fully matured adults than they are to pre-adolescents.