The US must stop giving young offenders life sentences without the chance of parole, human rights groups have said.
The law ignores youngsters' potential for change, Amnesty said
A report by Amnesty International and Human Rights Watch said such prisoners - numbering at least 2,225 in the US - must have access to parole processes.
The report says no more than 12 young offenders are serving life without parole in the rest of the world, where the punishment is largely outlawed.
The rights groups spoke to some 375 inmates and used data from many states.
The 157-page report, entitled The Rest of their Lives: Life without Parole for Child Offenders in the United States, was compiled over two years.
It found that 42 US states have laws allowing for offenders under the age of 18 to be sentenced to jail for life with no possibility of parole.
'Robbed of redemption'
Virginia, Louisiana and Michigan were found to be the most aggressive in imposing such sentences.
The practice is outlawed in many countries and by international law, under the UN Convention on the Rights of the Child.
The US and Somalia are the only two countries that have not ratified the treaty, the rights groups said.
Children too young to vote or buy cigarettes should also be considered too young to spend the rest of their lives behind bars, Human Rights Watch Senior Researcher Alison Parker said in a statement.
The executive director of Amnesty International in the US, William F Schulz, said the judicial system must be changed.
The courts, he warned, were in danger of becoming "assembly lines that mass produce mandatory life without parole sentences for children, that ignore their enormous potential for change and rob them of all hopes of redemption".
The report claims that an increasing number of children are receiving life without parole, even as the number of children convicted of serious crimes such as murder has fallen.
It found that the vast majority - 93% - of young offenders serving the sentence had been guilty of murder.
However, some 26% of youths sentenced to life without parole were guilty of "felony murder", where they were deemed accomplices to murder, even if they did not directly kill anyone.
The report cited the example of a 15-year-old prisoner, Peter A, who received the sentence because he had stolen a van used by two older accomplices who committed a double murder during a robbery.
Across the US, black youth were found to be 10 times more likely to receive life without parole than white youth.
In Pennsylvania, Hispanic youth were found to be ten times more likely to receive the sentence than their white contemporaries.
A spokesman for Mark Warner, the governor of Virginia, told the Associated Press news agency the punishment had widespread public support in the state.
Kevin Hall told the agency the governor believes young offenders should be able to receive life without parole "for crimes so heinous that prosecutors present that as an option".