I'm not a Neuro scientist. Professor Sarah-Jayne Blakemore is and her explanation of Synaptic pruning and adolescent brain development will always be better than mine.

Why do teenagers seem so much more impulsive, so much less self-aware than grown-ups? Cognitive neuroscientist Sarah-Jayne Blakemore compares the prefrontal cortex in adolescents to that of adults, to show us how typically "teenage" behavior is caused by the growing and developing brain.