The justices heard two hours of argument Monday in cases involving two Florida teenagers who are serving life prison terms.
Justice Ruth Bader Ginsburg said that "because of immaturity, you can't really judge a teenager at the point of sentencing."
But Justice Samuel Alito wondered why someone who commits rape, but doesn't kill the victim must get a second chance.
Joe Sullivan was sent away for life for raping an elderly woman when he was 13. Terrance Graham was implicated in armed robberies when he was 16 and 17.