The two decided cases - Miller v. Alabama and Jackson v. Hobbs - will affect 2,500 to 2,600 inmates in 28 states that mandate life in prison without parole for juveniles tried as adults and convicted of murder.
Pennsylvania has the largest population of teen lifers: 480, about 350 from Philadelphia, including a quarter of the 80 inmates nationwide serving life without parole for crimes committed when they were 14 or younger.
Writing for the majority, Justice Elena Kagan noted that a line of Supreme Court decisions since 2005 on juvenile defendants "make clear that a judge or jury must have the opportunity to consider mitigating circumstances before imposing the harshest possible penalty for juveniles."
"By requiring that all children convicted of homicide receive lifetime incarceration without possibility of parole, regardless of their age and age-related characteristics and the nature of their crimes, the mandatory sentencing schemes before us violate this principle of proportionality and . . . the Eighth Amendment."
Three dissenting opinions were filed by Chief Justice John G. Roberts Jr. and Associate Justices Antonin Scalia, Clarence Thomas, and Samuel A. Alito Jr. The dissents generally held that the court did not have the authority to second-guess state legislatures that imposed such punishment on juveniles.
"It is a great tragedy when a juvenile commits murder - most of all for the innocent victims," Roberts wrote in his dissenting opinion.
"Perhaps science and policy suggest society should show greater mercy to young killers, giving them a greater chance to reform themselves at the risk that they will kill again. But that is not our decision to make."
Juvenile justice advocates say the decision will likely lead to a review of the 480 Pennsylvania cases and creation of a legal mechanism mandated by the Pennsylvania Supreme Court through which juvenile lifers can present evidence showing that they deserve to be considered for parole.
Pennsylvania law does not permit parole for convicted murderers. Only the governor may commute a life term, and only after the state Board of Pardons unanimously recommends clemency.
"This does not say that 480 juvenile lifers will get out tomorrow," said Marsha Levick, deputy director and chief legal counsel at the Juvenile Law Center, a Philadelphia-based public interest law group that filed a friend-of-the-court brief with the Supreme Court on the Arkansas and Alabama cases.
Levick said the Pennsylvania Supreme Court had held pending the U.S. Supreme Court's decision a petition on behalf of juvenile lifers by her and Bradley S. Bridge, a lawyer with the Philadelphia public defender's office.
"The [state] Supreme Court has the power of immediate remediation in these cases," Levick said.
The nation's high court did not go so far as to ban sentences of life without parole for juveniles; it banned mandatory sentences that are part of a guilty verdict without a formal sentencing hearing.
Both Levick and Bridge acknowledged that a judge could still hold a sentencing hearing for a juvenile convicted of first- or second-degree murder, consider the defendant's mitigating evidence, and impose life without parole.
But, as Kagan points out in the majority opinion, a judge would likely impose that sentence only on "the rare juvenile offender whose crime reflects irreparable corruption."
The most vocal critics of the high court's decision were victim advocates.
The National Organization of Victims of Juvenile Lifers released a statement saying the decision will reopen the anguish that victim families suffered until the killer was convicted and sentenced.
"Victims' family members whose cases have been reopened by this decision will need resources devoted to supporting them through this process," said Bobbi Jamriska of the group's Pennsylvania chapter. "We call on everyone involved . . . to devise a system that can best serve not only the offenders but also victims and the public."
Monday's decision follows a series of Supreme Court rulings based on developing brain science that held that juveniles are markedly more immature than adults, and thus more likely to commit crimes without appreciating what they are doing, and are also much more amenable to rehabilitation.
In 2005, in Roper v. Simmons, the Supreme Court barred the use of the death penalty for juveniles. Five years later, in Graham v. Florida, the court struck down sentences of life without parole for teens convicted of crimes other than murder.
William J. DiMascio, executive director of the Pennsylvania Prison Society, the nation's oldest prisoner-advocacy group, said he hopes the ruling will spark a public discussion about mandatory sentences and life without parole for murder.
"It all comes down to the fact that there are too many people in prison for too long," DiMascio said. "The value of incarceration is fairly limited."
Contact Joseph A. Slobodzian at 215-854-2985, email@example.com, or @joeslobo on Twitter.