According to the Campaign for Youth Justice, which lobbies against the practice, some 200,000 youths are tried, sentenced or jailed each year as adults across the nation.
In December 1996, Dwayne Betts, who was 16 at the time, stole a car at gunpoint. After serving nine and a half years in five different Virginia prisons for carjacking, he is preparing to publish a book.
"I was fortunate because I think I was mature for my age. And I was smart enough to know that if you go to the prison and if you show you don't need anybody, they (other detainees) understood that you're on your own," the 28-year-old African-American man told AFP, explaining you had to be prepared to be a hard guy.
"But everybody don't know it," he added. Thrown into "a world ruled by adults," some adolescents "weren't old enough to cope with their own desires, to cope with the game that other people had."
One of Dwayne's former cellmates, now a 26-year-old, was also sentenced as an adult despite being a minor, and was jailed for 99 years.
Another former prisoner, Andre, 18, who spoke to AFP from the headquarters of the Campaign for Youth Justice in Washington on condition that only his first name be used, remains angry at the system that put him away.
"When you're locked behind the prison doors, it's like you forget about your own life, that is gone," he whispered slowly.
"When I first got into jail, I felt violated because you had strip in front of all the other men and squat and do all that in front of all the men," recalled Andre, who was freed a couple of months ago after serving his time for robbery.
"This is all new for me. I'm 16. This is my first offense and I don't even have a record. So I didn't know what was going on."
The teenager found an escape in reading and writing poetry, especially after being sent to North Dakota, thousands of miles (kilometers) from his family to serve the last months of his time.
He lowered his head. "I guess my judge thought I was such a bad person, he told me I was a menace to the society. I'm a menace to the society? This is my first charge I've ever committed!" he said.
The laws on trying juveniles vary from state to state. In some places for example such as Connecticut, New York and North Carolina, anyone aged 16 and above is automatically tried as an adult.
In some states it depends on the crime, and in Oregon it's up to the judge to decide whether the defendant should be tried as a minor or not.
"Most states passed laws in the 1990s to make it easier to try youth as adults. These state law changes were a result of predictions of increased youth crime which never materialized," said Liz Ryan, president of the Campaign for Youth Justice.
And the scars are hard to erase. Andre now has a felony on his record, and says after two years in prison, "I had trouble to get back in high school because once I had a felony on my record, they like treated me like a gangster.
"What they didn't understand is that this is my first offense. Everybody makes mistakes." Had Andre been tried as a minor, the records would have been sealed and no-one would have known that he had served jail time.
Tyrone, 16, who was also charged with robbery, explained that in adult detention facilities, "they don't really look at you, they push you in a cell, just they close the door."
In juvenile detention, however, "they come back they check, they look at you, you have three meals a day, sometimes four," said Tyrone.
"You are more privileged, you're not on your own," recalled Raymond, another teenage prisoner. "It wasn't that bad."
In adult detention, young prisoners "are frequently not provided with the appropriate services they need to address their risk and protective factors," said David Altschuler, a juvenile justice expert at Johns Hopkins University.
"They are housed with other mostly older offenders that are a bad influence, they may be victimized by adult offenders and they are released back into the community under adult parole or probation that offers little supervision."
--------------------------------------------------------------------------------------------------------------------------