It is generally believed that our brains undergo a massive reorganization between the ages of 12 and 25. The axons (connectors) between neurons become better insulated with myelin. More heavily used synapses (junctions) strengthen, and the little used ones wither. The result is a faster, more sophisticated and more specialized brain. We become more balanced and more sensible. While the brain is thus maturing, we may be easily distracted, excessively sensitive, take unnecessary risks, etc.
New research has illuminated the teenager brain from a different angle (National Geographic, October 2011, p. 36-59). Recent research have found that the teenager brain is more sensitive to dopamine. As a result, it learns quicker, and values rewards (against taking risks) more than the adult brain. It is also more sensitive to oxytocin, making social connections more rewarding. Hence they prefer the company of their peers more than adults.
All these make teenagers do more foolish and dangerous new things with young friends.
On the other hand, they also make them more interested in making friends, more adaptive, more likely to leave home to strike out on their own, and invest in the future rather than the past. These are traits that make us more social and more successful in life.