Adolescent Brains – risk taking, and peer pressure.
In the USA, I have worked on 5 cases in the past year where an inmate convicted of a non-homicide offence, and sentenced to life without parole has been given the possibility of appealing the decision following the case of Graham v. State of Florida. There will be much more on this to follow, but in essence the ruling of the Supreme Court in Graham was that as juveniles are not fully developed, they should be given the opportunity in adulthood to show that they have changed and are no longer a danger to society.
This article from the National Geographic Magazine, although not scientific, makes interesting reading on this very point..