Illinois’ top court will start a new term this week by grappling with the legality of de facto life sentences for young adults.The Illinois Supreme Court is slated to hear arguments Tuesday against a 76-year prison sentence for a man convicted of murder shortly after he turned 18.An appeals court vacated the sentence for violating a state constitutional provision that says penalties should be meted out not only based on the severity of the crimes, but with the goal of restoring people to “useful citizenship …