PHILADELPHIA — Hundreds of Pennsylvania inmates serving life in prison for juvenile crimes could seek parole after 20 years under a plan proposed Wednesday by juvenile law advocates.

The U.S. Supreme Court recently outlawed mandatory life-without-parole terms for young killers, deeming it cruel and unusual punishment.

That leaves trial judges in Pennsylvania in a quandary, because the state has only two sentences for first-degree murder: death row or mandatory life without parole. Both have now been outlawed for juveniles.

Lawmakers plan to rewrite the Pennsylvania crime code for future cases. But defense lawyers want the state Supreme Court to give judges guidance in the interim, and to address the fate of hundreds of juvenile offenders already in prison for life. Some are now in their 70s.

In arguments Wednesday, Marsha Levick of the Philadelphia-based Juvenile Law Center suggested converting all the sentences to 20- to 40-year terms, the current sentence for third-degree murder.

Justice Seamus McCaffery complained that a 17-year-old killer could get out at 37.

Levick countered that culpability can vary widely, from a 14-year-old caught in a getaway car with a gunman to an older teen who plans and carries out a murder.

In more severe cases, she said, a judge could run the murder sentence consecutive to other counts involved, such as the 10 to 20 years for felony robbery or 20 to 40 years for conspiracy. “The reality in many of these cases is these individuals will serve very lengthy sentences and, frankly, will never max out,” Levick said.

Justice Max Baer seemed inclined to grant each inmate a new sentencing hearing, so judges can weigh each one’s background, intellect, degree of culpability and other factors.

The justices did not indicate when they would rule.