The changes would allow some of these offenders to get parole hearings as soon as15 years after their incarceration, and even sooner in some cases in which, for example, the offender did not pull the trigger or otherwise kill the victim. Parole boards would still have discretion to keep them behind bars for life.

The administration proposal would require that first- and second-degree murder cases for teenagers aged 14 to 17 be tried in juvenile court instead of adult court, as is currently required. It would also broaden the jurisdiction of juvenile court judges over other offenses, changing the definition of a juvenile from 16 and under to 17 and under.

Juvenile judges would still have the discretion to sentence young offenders to life without parole for first-degree murder, but only after written findings that specific criteria —related to the defendant’s intellectual development and capacity for rehabilitation and risk to the public — have been met.

Patrick’s proposals, which come in response to a US Supreme Court ruling in June that said states could no longer automatically detain all teen murderers for life without parole, will serve as a key stake in the ground on Beacon Hill this year in a fierce debate over juvenile justice.

Lawmakers have offered several alternative proposals, including at least one that would keep teen murderers behind bars for a minimum of 35 years.

Many defense attorneys and others who advocate on behalf of young offenders have pushed for quicker parole eligibility, citing a growing body of evidence that teenagers lack the brain development to fully comprehend their crimes, and that they are better candidates for rehabilitation than older offenders.

“Governor Patrick has proposed this legislation to create a fairer justice system for our youth and for our Commonwealth,” said Gail Garinger, the state’s official child advocate who advises Patrick, in a statement released by the administration. “It is wrong to give up on any young person without considering the circumstances of his crime, his life experiences, and without asking whether he is capable of being rehabilitated before we sentence him to spend the rest of his life in prison without the possibility of parole.”

But many prosecutors and victims’ groups worry that if sentences become too lenient, they will not reflect the gravity of the crimes.

Middlesex District Attorney Gerry Leone said in a statement that, in the absence of life without parole, “those who kill with deliberate premeditation or extreme atrocity and cruelty” should serve at least 35 years before they are eligible for release.

“For a 15-year-old killer, parole eligibility at age 50 limits the risk of recidivating and still provides a life expectancy of 20 years, where the victim has none,” he said.

Current Massachusetts law was passed in 1996, soon after 15-year-old Eddie O’Brien was convicted of killing his friend’s mother, slashing her nearly 100 times in her Somerville home.

The law requires life without parole for all offenders convicted of first-degree murder, aged 14 and older. But last year’s Supreme Court decision, Miller v. Alabama, deemed automatic life without parole sentences for juveniles, imposed in 29 states, unconstitutionally cruel and unusual punishment.

Other states affected by the ruling have so far proposed a range of laws, but only a few have enacted them, according to Marsha Levick, deputy director of the Juvenile Law Center, a Pennsylvania-based advocacy group. Pennsylvania law provides minimums of 25 and 35 years to life for first-degree murder, depending on whether the defendant is older or younger than 15. California allows for parole after 15 years in some cases, but requires at least 25 years served before most offenders are eligible, she said in an e-mail.