Children and young people who kill do not always act with the mental acuity expected of someone who has reached full maturity, yet when they are charged with murder or manslaughter they are often tried as adults. This contradicts the basic principle of criminal responsibility that criminalisation is based upon moral blameworthiness. A conviction for murder requires at least a conscious appreciation of a real risk of death. Recent research by developmental neuroscientists shows that adolescence is a developmental stage and that the adolescent brain is not capable of the same level of reasoning expected in the average adult. This article suggests that the criminal justice system can adapt to these recent advances in knowledge via specific defences of a diminished capacity class, or by allowing evidence of general adolescent brain development to support defence arguments that a young accused did not form the mens rea required for murder.