Molly J said of her time in solitary confinement:

"[I felt] doomed, like I was being banished … Like you have the plague or that you are the worst thing on earth. Like you are set apart [from] everything else. I guess [I wanted to] feel like I was part of the human race – not like some animal."

Molly was just 16 years old when she was placed in isolation in an adult jail in Michigan. She described her cell as being "a box":

"There was a bed – the slab. It was concrete … There was a stainless steel toilet/sink combo … The door was solid, without a food slot or window … There was no window at all."

Molly remained in solitary for several months, locked down alone in her cell for at least 22 hours a day.

No other nation in the developed world routinely tortures its children in this manner. And torture is indeed the word brought to mind by a shocking report released today by Human Rights Watch and the American Civil Liberties Union. Growing Up Locked Down documents, for the first time, the widespread use of solitary confinement on youth under the age of 18 in prisons and jails across the country, and the deep and permanent harm it causes to kids caught up in the adult criminal justice system.

Ian Kysel, author of the 141-page report, interviewed or corresponded with more than 125 young people who had spent time in solitary as children in 19 states. To cope with endless hours of extreme isolation, sensory deprivation and crippling loneliness, Kysel learned that some children made up imaginary friends or played games in their heads. Some hid under the covers and tried to sleep as much as possible, while others found they could not sleep at all.

"Being in isolation to me felt like I was on an island all alone dying a slow death from the inside out," a California teen wrote in a letter to Human Rights Watch.

One young woman, who spent three months in solitary in Florida when she was 15, described becoming a "cutter" while in isolation:

"I like to take staples and carve letters and stuff in my arm … Each letter means something to me. It is something I had lost."

She started by carving into her arm the first letter of her mother's name. Another girl who cut herself in solitary said, "because it was the only release of my pain."

In fact, solitary confinement has been shown to cause severe pain and psychological damage to the tens of thousands of adults who endure it every day in American prisons. On children, the report states, the practice has a "distinct and particularly profound impact." Because of "the special vulnerability and needs of adolescents, solitary confinement can be a particularly cruel and harmful practice when applied to them." This is all the more true because for many of these kids, "developmental immaturity is compounded by mental disabilities and histories of trauma, abuse, and neglect."

Yet, prisons and jails commonly use isolation as punishment for violating prison rules, including both violent and nonviolent infractions. One boy who entered a Colorado jail at age 15 said the guards doled out stints in solitary for crimes that would, in any other setting, be deemed normal adolescent behavior:

"15 days for not making the bed; 15 days for not keeping the cell door open; 20 or 25 days for being in someone else's cell."

On Rikers Island in New York City, more than 14% of adolescents between 16 and 18 spent some period in "disciplinary segregation". This despite the fact that nearly half of all adolescents on Rikers have been found to have a "diagnosed mental disorder".

Other kids are isolated as a form of "protective custody", because they are vulnerable to physical or sexual abuse. Even though they are being locked down "for their own good", many receive no educational or rehabilitative programming while in solitary, and some are barred from seeing their families.

Still, other children are placed in solitary confinement for "treatment" purposes, especially after threatening or attempting suicide – even though isolation has been shown to sharply increase the risk that prisoners will take their own lives.

"There is nothing to do so you start talking to yourself and getting lost in your own little world. It is crushing," said Paul K, who spent 60 days in solitary when he was 14.

"You get depressed and wonder if it is even worth living. Your thoughts turn over to the more death-oriented side of life."

No one knows precisely how many children live in these conditions, since many state and local correctional systems do not keep such data. But the overall rate of solitary confinement in American prisons is thought to be between 3% and 5%, and anecdotal evidence suggests that, in some systems, children may be isolated at even higher rates than adults. Given that nearly 100,000 youth under the age of 18 pass through adult prisons and jails annually, there exists the staggering possibility that thousands of children are spending time in solitary confinement each year.

Liz Ryan, who directs the Campaign for Youth Justice, points out that 20 states have laws requiring that juveniles be kept apart from adult prisoners. Yet most of the nation's 3,000 jails lack dedicated facilities for children – leaving them with no alternative but to place kids in solitary. A majority of people in jail are there awaiting trial, which means many children in solitary have not even been convicted of a crime.

In addition, Ryan said, "A kid could be held in jail not because there is a risk to public safety, but because they don't have the resources to make bail." So the racial and class disparities endemic to the criminal justice system are likely reflected in the population of children languishing in isolation. Ian Kysel said in an interview:

"I think one of the greatest impediments to change is trying to unravel the policy issue that is at the root of this problem: a criminal justice system that treats kids as if adults without providing resources or guidelines for their care,"

For this reason, Human Rights Watch and the American Civil Liberties Union recommend that state and the federal governments "prohibit the housing of adolescents with adults, or in jails and prisons designed to house adults". However, "regardless of how they are charged and held," Kysel says unequivocally:

"We need to ban the solitary confinement of young people across the board. There is simply no reason that a child or adolescent ever needs to be held in a cell, alone, for 22 let alone 24 hours at a stretch."

For this to happen, though, the American public will need to accept what numerous international bodies have already concluded – that solitary confinement is cruel, inhuman and degrading treatment, and clearly rises to the level of torture when levied against vulnerable populations, including children.

"If my story can stop another kid from coming" to solitary confinement, one Florida teen wrote, then, "Hopefully my pain serve[s] some purpose."

• Editor's note: this article originally referred to the Campaign for the Fair Sentencing of Youth; this was amended to the Campaign for Youth Justice at 10am (ET) on 12 October 2012