Amends the Unified Code of Corrections. Provides that, except for certain types of first degree murder, the court may, in its discretion, sentence a defendant who was under 21 years of age at the time of the commission of the offense to a sentence that is less than the applicable minimum determinate sentence of imprisonment for the offense authorized by the Code. Provides that the court, at the sentencing hearing, shall consider certain mitigating factors when a person commits an offense and the person is under 21 (rather than 18) years of age at the time of the commission of the offense. Effective immediately.