The U.S. Supreme Court has already said — twice — that teenage criminals are not the same as their adult counterparts.

Now the high court is expected to rule, perhaps today, whether it is constitutional to sentence teenage killers to life without parole, considering that teenagers’ brains are not yet fully developed.

Life without parole is a punishment Georgia and 38 other states have imposed on 2,700 juvenile killers, and the court ruling could impact the punishment of Jonathan Bun, who was 17 when he murdered Clayton County sheriff’s Deputy Rick Daly last July.

Depending on how the Supreme Court rules, Bun’s lawyer is due in court Thursday to argue that it would be cruel and unusual punishment, a violation of the Eighth Amendment to the U.S. Constitution, to sentence the teenager to life without parole for something he did as a juvenile.

Clayton County District Attorney Tracy Graham Lawson disagrees. “Once in a while there is a young person so mean that the only way to protect society is to lock them away for the rest of their lives,” she said.

“The question is how big a pound of flesh do you need?” said Stephen Bright, senior counsel for the Southern Center for Human Rights and a law professor at Yale University. “We know in the teenage years the child’s brain is not fully developed, especially for anger control and impulsive behavior.”

Opponents of the sentence say some people can be redeemed and should have a chance at parole, at least after they have spent decades in prison — a minimum of 30 years for anyone sentenced to life.

Advocates of life without parole for criminals of any age say the penalty is designed for those who are likely to always be so dangerous that they should never again walk free.

In 2005, the Supreme Court put the death penalty off-limits for murderers younger than 18, which left prosecutors with only the option of life in prison without parole for juveniles who commit crimes so heinous that life with the possibility of clemency is not enough.

In 2010 the justices again looked at punishment for teenage criminals. At that time, the court ruled that sentencing a teenager to life without parole for a crime that didn’t involve a death violated constitutional protection from cruel and unusual punishment.

Now the justices are examining the cases of two 14-year-old killers — one in Alabama and the other in Arkansas — to determine whether a juvenile should be sentenced to life without parole for any crime.

Emory law professor Kay Levine said previous Supreme Court decisions suggest the justices have a “deepening appreciation for scientific evidence of the developing juvenile brains. The brain does not develop until around the age of 25. As a result, the court is increasingly uncomfortable with enforcing such harsh punishment ... on people whose brains have not formed.”

Of the 39 states that allow life without parole for juveniles, Pennsylvania has the most — 450 inmates — who came into the prison system for crimes they committed when they were teenagers.

Georgia has sentenced far fewer teenagers to life without parole. Until recent years, state law allowed prosecutors to ask for life without parole only when the circumstances called for the death penalty, which they could not seek for juvenile defendants.

Of the 31 Georgia juveniles sentenced to life without parole, one was 13 and two were 14 when they killed. Only one of the 31 is still a juvenile, according to Department of Corrections data.

Lacey Aaron Schmidt, now 16, was sentenced in March. He was 14 when he shot classmate Alana Calahan in the head and hid her body in Columbia County woods.

The girl’s parents called Schmidt a monster and a hateful beast.

Still, Levine raises the question: “Does it make sense to condemn someone for the rest of his or her life for a dangerous, irrational decision made as a juvenile?”

“You cannot predict how a teenager is going to be in 20, 30 years,” Bright said.

But there must be consequences, said Bruce Holmes of Rex, whose 17-month-old great-grandson was murdered in 2009 by his teenage father.

Brain development is “not an excuse,” Holmes said. Otherwise, “it would become open season for people to commit crimes knowing they won’t have to serve any big time for it. They aren’t thinking with a full deck here. You’re opening the door for a lot of stuff.”

According to legal experts, the Supreme Court’s possible decisions are:

● Let the 39 states’ laws stand.

● Determine that under no circumstances should children be sentenced to life without parole.

● Rule that a judge must take into consideration the age and the circumstances before choosing a sentence of life without parole.

If the justices choose the third option, Bright said, the 31 in Georgia prisons could still be resentenced to life without parole after a judge takes their age into account.

If the Supreme Court decides to allow the penalty of life without parole for juveniles, said Lloyd Matthews, Bun’s defense attorney, “taxpayers will be paying until he dies. ... He did kill someone and he needs to be punished. But he is not as culpable.”

That makes no difference to Holmes. In the case of the killer of his great-grandson, he said, life without parole “is not enough time for the crime.”