Amends the Unified Code of Corrections. Provides that, on or after the effective date of the amendatory Act, when a person commits an offense and the person is under 21 years of age at the time of the commission of the offense, the court, at the sentencing hearing, shall consider certain additional factors in mitigation in determining the appropriate sentence. Provides, that notwithstanding any other provision of law, if the defendant is under 18 at the time of the commission of the offense and convicted of first degree murder and would otherwise be subject to sentencing under certain provisions, the court shall impose a sentence of not less than 40 years of imprisonment. Provides that, in addition, the court may, in its discretion, decline to impose the sentencing enhancements based upon the possession or use of a firearm during the commission of the offense.