NEW YORK — The United States was singled out Monday by a United Nations expert on torture for being the only country in the world that continues to sentence children to life in prison without parole.

“The vast majority of states have taken note of the international human rights requirements regarding life imprisonment of children without the possibility of release,” Juan Méndez, the United Nations special rapporteur on torture and other cruel, inhumane or degrading treatment or punishment, said in his report, before noting that the United States is the only country to continue the practice.

A sentence of life without parole means life and death in prison — a practice considered cruel and inhumane punishment for juveniles under both international and U.S. law.

“Life sentences or sentences of an extreme length have a disproportionate impact on children and cause physical and psychological harm that amounts to cruel, inhuman or degrading punishment,” the report reads.

Dr. Louis Kraus, the chairman of the juvenile justice reform committee at the American Academy of Child and Adolescent Psychiatry, called the practice “a devastating process to even conceptualize.”

“These kids have not developed. These are eighth-graders and, in some states, younger than that,” he said.

Issuing life sentences for children is banned under numerous international laws, including the International Covenant on Civil and Political Rights, the Convention Against Torture and the U.N. Convention on the Rights of the Child — which the U.S. and South Sudan are the only two states to have signed but not ratified. Also, a U.N. oversight body has found that the sentence violates the Convention on the Elimination of All Forms of Racial Discrimination, since youths of color are more likely to receive the sentence than white offenders.

The U.S. Mission to the U.N. did not return a request for comment by time of publication.

“The toughest part is that the crimes children might have committed, as devastating as they may have been, are really in unformed brains,” said Kraus. “These teenagers are not the same as their adult counterparts will be. Many of them are not going to be that same person. They're going to show greater insight, better empathy, less impulsivity, better reasoning ability in terms of understanding the short- and long-term ramifications of their behavior.”

The U.S. Supreme Court, in the case of Miller v. Alabama in 2012, outlawed mandatory sentencing of life without parole for children under 18, arguing that the sentence violated the Eighth Amendment, which prohibits cruel and unusual punishment.

Delivering the opinion of the court, Justice Elena Kagan wrote, “Mandatory life without parole for a juvenile precludes consideration of his chronological age and its hallmark features — among them, immaturity, impetuosity and failure to appreciate risks and consequences. It prevents taking into account the family and home environment that surrounds him — and from which he cannot usually extricate himself — no matter how brutal or dysfunctional.”

Still, while the Supreme Court has ruled that sentences of mandatory life without parole are unconstitutional, judges at the state level can make sentencing decisions based on the circumstances in which the crime was committed. About 2,500 people in the United States are currently serving life sentences without the possibility of release for crimes they committed as children.

“The big issue is whether that decision from the Supreme Court [on mandatory sentencing] has a retroactive effect so that persons who are serving life in prison without parole can benefit from that,” said Steven Watts, the senior staff attorney at the American Civil Liberties Union human rights program.

Fourteen states and the District of Columbia have banned life sentences without parole for juveniles; Hawaii and West Virginia joined the roster in 2014. Earlier this month, the American Bar Association called for a complete end to life without parole for children.

“In the 1990s, [courts] increased the numbers of offenses for which children could be sentenced as if they were adults. They were charged, prosecuted — as young as 14 — charged, prosecuted, convicted and sentenced as if they were adults,” said Watts, “That comes from a debunked theory from back in the 1990s that there are these superpredators, incorrigible children, that it was built into their DNA that they would do the wrong things.”

“We’re still living with those laws, enacted at that time, and it happens to a degree in the U.S. that doesn't happen anywhere else in the world,” he said.

Amnesty International USA has for years championed the case of Jacqueline Montanez, jailed for life in 1992 at the age of 15, without the possibility of release. She was convicted in adult criminal court for the death of two members of a rival gang and given a mandatory life sentence without parole. She pleaded not guilty at the time but says she has since accepted full responsibility for her involvement in the murders.

“When children come into conflict with criminal law, our primary objective as a society should be maximizing their potential for successful reintegration into society,” Steven Hawkins, executive director of Amnesty International USA, said in a January 2015 letter to then-Gov. Pat Quinn of Illinois, asking for clemency for Montanez. “To deny the possibility of release is to deny the human capacity to change and is utterly incompatible with the basic principles of juvenile justice.”

Montanez, like many juveniles sentenced to life in prison, was exposed to violence and abuse at home. According to a 2012 report by the Sentencing Project, many individuals incarcerated for life as children experienced high rates of violence, abuse and economic disadvantage growing up.

“We know that this practice is also unfairly imposed upon our most vulnerable citizens — those who have already been failed by many of the systems that are supposed to protect them,” said Jody Kent Lavy, the director and national coordinator of the advocacy group the Campaign for the Fair Sentencing of Youth. “In addition, there are significant racial disparities in the use of the sentence, with black youth sentenced to life without parole at a per capita rate 10 times that of white teens.”

But it’s not only children in the criminal justice system that concern the U.N.’s Méndez; any form of detention is detrimental to the health and well-being of children, including immigration detention centers.

In a statement released ahead of the report, he said, “Detention of children based on migration status is never in the best interests of child, is grossly disproportionate and constitutes ill treatment.”

Experts say immigration detention can have profound negative effects on children’s mental health and development. “That’s partly because they are deprived of the kind of normal, everyday experiences and opportunities that they need developmentally,” said Dr. Sarah Mares, a child and family psychiatrist and medical consultant to the Australian Human Rights Commission’s recent inquiry into children in immigration detention.

“They are exposed to very high levels of adult distress, so things that can be protective for children and young people in everyday life when they’re facing adversity are not available for kids that are detained,” she said.

“Particularly for children, there’s a very significant link between deterioration in their mental health and functioning and the length of time in detention,” said Mares. “That’s why it’s clear that detention for children needs to be for the minimum possible time.”

According to the Global Campaign to End Child Detention, over 67,000 unaccompanied children and 2,000 families were detained in the U.S. in 2014.