And there in front of you is a person looking up, with a nervous smile. She has blue eyes, hazel brown hair and freckles. Her prison number is stamped across her back: 599905. Nicole Ann Dupure. Height: 5ft 2in. Weight: 140lb. Date of birth: July 8 1986. Earliest release date: Life.

When she was sentenced, the judge ruled that the time she had spent in jail awaiting trial - 264 days - should be credited against her term of incarceration. What does that mean? Nobody can predict when Dupure will die, so nobody can calculate when to let her out. Her sentence demands she stay in the Robert Scott Correctional Facility, the main women's prison within the state of Michigan, for the rest of her natural life. She will never have the chance to demonstrate her remorse or convince anyone she has reformed: it is stipulated she is not entitled to parole.

Dupure was 17 when the crime for which she is convicted took place. She is one of 2,270 juveniles across the United States who were sentenced to life without parole, a punishment second only in severity to the death penalty. All were under 18 when they committed the crimes. Six of them were 13, and 50 of them were 14 - an age at which US law forbids them to drive a car, give medical consent, vote, leave school, sign a contract, drink alcohol in a bar, serve on a jury, be drafted in the army, live away from home. Yet they were tried as adults in an adult court and given no possibility of a second chance.

In Michigan, Dupure is one of 307 such inmates, the third highest number in any American state after Pennsylvania and Louisiana. She describes her prison day. She gets up at 4am to work in the kitchen. She does a 40-hour week, earning 18 cents an hour. When she asked the prison authorities if she could take a business vocational course, she was turned down on the grounds that as she will never be set free there is no point learning skills geared to rejoining the outside. It's not exactly what she intended to do with her future when she was a teenager, she tells me. At school she had aspirations to become a medical lab technician, specialising in the treatment of heart defects. Her background was far from typical for a lifer - no criminal record, no history of alcohol or drug abuse, a high school graduate with mainly B grades. Her next step was to be college.

A chance encounter when she was 17 changed everything. She was working in the holidays to earn petrol money at a grocery store near her home in Michigan's St Clair County. There was a 19-year-old working there called William Blevins who was funny and charismatic - they started dating. "I wasn't able to see the warning signs. My mum did. She said he didn't seem like a good kid and I shouldn't be around him as he would bring me down. I didn't listen to her. I thought like any teenager that she just didn't want me to have a boyfriend."

When Blevins was thrown out of his home by his parents, Dupure, by then pregnant, left home to be with him. "I just didn't want him to be alone," she says. They went looking for a motel room to rent. On April 23 2004 they stopped off at Big Boy, a fast-food restaurant she knew well because it was near the apartment of her great-aunt's best friend, Shirley Perry. Perry, who was 89, used to babysit for Dupure when she was very young; Dupure and Blevins had been to her flat several times, offering to help her with shopping and odd jobs.

At this point the official version parts company with Dupure's. In court, the prosecution alleged that the teenagers plotted together to kill Perry for her money. They took just $30 from her flat to pay for motel fees and two milkshakes at Big Boy. Dupure actively participated in the murder, striking the old woman on the head with a cooking pot and fetching the kitchen knife Blevins used to kill her.

Dupure insists she was not in the apartment at all, but waited in the restaurant, oblivious to the events unfolding, while Blevins went off on his own. What is certain is that Blevins murdered the old woman, stabbing her several times and strangling her. Under police questioning he admitted it, saying he acted alone. But shortly before he went on trial he changed his evidence and put Dupure alongside him at the scene of the murder. In return, the prosecution agreed he should be given the lesser charge of second-degree murder and avoid lifelong incarceration. Under cross-examination, he conceded to the jury, "I never had intentions to pin it on her until I ran out of options."

Blevins got 20 to 50 years, with the hope of reducing his sentence through good behaviour. Dupure got life without parole, with no forensic evidence tying her to the crime and entirely on the strength of Blevins' testimony.

Dupure has just turned 21 but she still looks 17. She was told about life without parole for the first time when she entered prison - "You never go home." She spent much of her first year crying, she says. The prison doctor put her on Prozac but she stopped taking it; as she puts it, "I'm depressed because I'm in this place, not because I'm depressed." Instead, she sees the prison therapist once a month. She finds talking to the therapist helpful - she can't be open with other prisoners about her fears and despair, as it would be taken as a sign of weakness. In prison, "you have to pretend to be strong".

"The difficult bit is blocking out the thought that I'm here for ever. You can only do that for so long, and then you break down. Something hits you. Somebody will say, 'I'm glad I haven't got life' and it will get you. Or one of the friends you made will leave for the outside and that hurts, so you stop getting close to people." She was recently sent a questionnaire by lawyers conducting a survey of juvenile prisoners on life without parole. Asked what was the most difficult challenge she faced inside, she wrote, "Trying not to feel alone."

Michigan is one of 41 states in America that allows children under 18 to be imprisoned for the rest of their lives. The US is among a tiny minority of countries (Somalia is another) that have refused to sign up to the UN Convention on the Rights of the Child that expressly forbids the practice. According to Amnesty International and Human Rights Watch, only three other countries - Israel, South Africa and Tanzania - mete out the sentence and they have collectively just 12 prisoners serving it.

Technically, a child of any age could be incarcerated for life in Michigan for first-degree murder. Above the age of 14, suspects can be placed directly into the adult court system. At that point, even the judges' hands are tied. If a child is convicted in an adult court of a range of serious offences - taking part in a robbery that leads to murder, say - they must automatically be given life without parole, even where the judge feels that is inappropriate.

That's what happened to Matthew Bentley. Prisoner 271014. Height: 5ft 4in. Weight: 135lb. Date of birth: October 4 1982.

Bentley broke into a house in Michigan's Colfax township on September 2 1997 looking for a car to steal and some ready cash. By ill luck, he chose a house where the owner was a gun enthusiast and had 25 handguns and rifles on the property. Bentley had no experience with firearms, but when he came across them in the hallway, he grabbed a couple of handguns along with a bottle of Coke. The woman of the house, Betty Bardell, surprised him when she walked into the hallway shouting, "What the hell are you doing here? I'm going to call the police!"

Bentley pointed one of the guns at her and pulled the trigger. The bullet hit her in the left shoulder, and she fell to the floor bleeding. He told me what happened next: "I stood over her and I spoke to her and I said, 'I'm sorry. If I could I would call the police to get you out.' I cried a little bit. And then I left."

Bardell bled to death on the carpet.

There is nothing innocent about Bentley. He has never disputed what he did. He was aware immediately he pulled the trigger that he had done a terrible thing. "Have I felt remorse? I felt sorry from the first second and I always have done." The only mitigation was his age and his background. When he murdered Bardell, Matthew Bentley was 14. His legal file contains details of his immediate family ...

Father: In jail for sexually abusing a relative. Mother: Alcoholic; facing trial for receiving stolen property. Half-brother: In jail for raping a family member. Sister K: Alcoholic. Sister T: History of arrests for domestic assault. Half-sister C: In mental hospital.

Bentley's parents, Terry and Debra, were serial divorcees. They married in 1975 and divorced two years later. Terry took a second wife, Grace, in 1978. They divorced a year later. Terry remarried Debra in 1980, and she gave birth to Matthew in 1982. They divorced for the second time in 1989. Terry remarried Grace in 1991, by which time he was already in jail as a sex offender. Matthew's childhood was as you might expect in the circumstances. He was a straight E grade student. He was regularly suspended for truancy and often in trouble with police. He started smoking dope aged 10. He was in and out of children's homes, and at the time he murdered Bardell he was on Zoloft for depression and Dexedrine for hyperactivity.

The jury took just an hour to find him guilty in July 1998. He was 15 by then. One of the jurors told the local paper that "a couple of the older ladies on the jury broke down when we got in the jury room. They had feelings for the Bentley kid and they said they had grandchildren about his age."

Judge Richard Knoblock gave him the only sentence available to him under Michigan law. But he made his feelings clear. "I don't agree with the legislature that he should be sent away for the rest of his life without parole," the judge said. "It was a horrendous crime he committed and certainly he should be punished very severely. I'm just not confident in having to lock him up for ever."

Bentley is now 24. He says he has spent the past nine years in jail first in denial, then in deep despair, and now in some degree of reconciliation with his fate. He has stopped fighting in prison and no longer gets punishment cards. He has thought, too, about why he did what he did when he was 14. "I was a child trying to be a man. I tried to step up to the roles that were missing in my home. I thought I'd get away with it. Even that was childish."

There is another document in his file, a poem that he wrote about his time inside. Part of it reads:

"Everyday that goes by,

You know you have to die

before you finish your time.

Suddenly you're all alone

and 50 or 60 years is too damn long.

Isolated in a world turn cold."

There are only three ways that prisoners put away for life as juveniles can hope to see the outside again. They can win an appeal, by proving there was a flaw in the trial process - they cannot challenge the sentence itself. They can receive a pardon from the governor of Michigan - except the governor has never pardoned a juvenile lifer. Or the state assembly could pass legislation outlawing the practice, and implement it retroactively. Lawyers working on many of the 307 cases have pulled together a bill that would do just that, which they hope will go before Michigan's lawmakers this autumn.

Deborah LaBelle, a leading lawyer who is supporting the bill on behalf of the American Civil Liberties Union, says she is as hopeful as she has ever been that the legislation might pass. "Sending someone to prison is partly about public security and partly about punishment. People are coming to understand that child prisoners should have a chance to prove they no longer pose a risk. And on punishment, then surely having a person spend more of their life in a prison cell than they had lived as children on the outside has to be sufficient even for the most unforgiving of people."

LaBelle has been careful to involve victims of juvenile crime and their families in the debate about changing the law, and several victims' families have privately offered their support. "They say that what happened was horrible and has devastated them, but they do not want the knowledge that the child who committed the crime will stay in jail for ever to rest on their conscience."

Kevin Boyd. Prisoner 251328. Height: 5ft 7in. Weight: 170lb. Date of birth: September 26 1977. He was 16 when, on August 6 1994, he helped his mother murder his own father. Like Matthew Bentley, Boyd came from a disturbed home. His mother, Lynn, was addicted to prescription and illicit drugs, his father, Kevin, was an alcoholic who regularly beat him. His parents separated on Boxing Day when Boyd was 11 and his mother went to live with another woman. Boyd went to 10 different schools before he dropped out at 15. He received psychiatric treatment and was in hospital after a suicide attempt.

On the night of the murder, Boyd's mother, high on drugs, met him at a Burger King and asked him for the keys to his father's flat, saying she was going to kill him. He handed over the keys. The next morning Boyd went to his father's flat and, hearing no one inside, forced open the door. Kevin senior was slumped in his easy chair. He had been bludgeoned with a baseball bat and stabbed 23 times.

Boyd was interrogated by police for eight hours. He told them he had handed over the keys and that was all. Then a second team of officers questioned him. They turned off the tape recorder, and kept repeating to him the mantra, "The truth will set you free."

"Every time I tried to tell them what happened, they shouted me down. 'No, you didn't do that!' This sounds totally irrational, I know, but after hours of that, I thought if I told them what they wanted they would let me out and it would all go away." He confessed to having been the one to stab his father 23 times, and was given life without parole.

Boyd has contemplated his actions and its consequences a great deal over the past 12 years in jail. He is writing an account of his childhood, the murder and his subsequent imprisonment. Though he protests that he was not the killer, he still holds himself wholly to blame for giving his mother the keys and thinks it was right that he was sent to prison for many years. "I'm not innocent. I was responsible for his death. I could have said no to my mother. I could have picked up the phone to warn my father. Anything. But I didn't, and I am suffering the consequences."

He says he has made that phone call to his father a "million times". "I think about it every night before I go to sleep." He has had a clean prison record for six years and leads a pretty solitary existence. He once had a pet, an injured meadow vole he found in the exercise yard that he nursed and then let go. He hasn't had a human visitor for 10 years, though he does correspond with his mother who is a lifer in the same prison as Dupure. He finds comfort in playing guitar, jogging around the yard and writing his memoir. In one chapter he says sorry to his father. "If I could change it, I would die in your place, just to hear your rare but contagious laugh one more time. Dad, I am so, so sorry. For what it's worth, I always loved you. I always loved you both."

Donald Logan. Prisoner 132850. Height: 5ft 5in. Weight: 135lb. Date of birth: June 23 1954. He was tried and convicted twice for the murder of a paperboy, Thomas Eldridge, who went to his school. They were both 16. At the first trial, Logan, who is black, was found guilty by 12 white jurors. His lawyers appealed on the grounds that the racial composition of the jury was prejudicial, and a retrial was ordered. In the second trial there were 11 white jurors, including two who were members of whites-only organisations. The 12th juror was black, but during the course of the hearing it emerged that she was the aunt of the prosecution's key witness who was giving evidence against Logan in exchange for a reduced sentence.

Logan's case illustrates two key statistics about juvenile life without parole. Of the 307 prisoners in Michigan on that sentence, 69% are black, compared with 15% of the state's population as a whole. A study by Human Rights Watch and Amnesty also found that more than one in four of the juveniles incarcerated for ever was convicted of "felony murder" - serious crimes during which someone is killed yet where the juvenile did not personally or directly cause the death.

The prosecution case against Logan was that he identified the paperboy to a gang of his elder brother's friends who had robbed Eldridge the previous week and wanted to prevent him giving evidence against them. Logan was alleged to have acted as lookout when two of the gang members shot the boy. It was never alleged he had pulled the trigger himself or even held a gun. "I killed nobody," Logan said. "The guys asked me who was the paperboy. I was the one who pointed him out. That's all I did."

At the trial, a psychologist who examined Logan said that though he was 17 by then, he had the level of understanding of a 12-year-old. The pre-sentence investigation described him as being "a failure in almost everything he ever tried" and he was labelled a "retard". As soon as he was arrested, aged 16, he was placed in an adult jail where he faced physical and sexual harassment from older prisoners. "That's one of the things I'd preach if I ever get out: never send a teenager to adult prison. They are just like a little animal who will get eat up the minute they arrive," he told me.

In the early 90s, Logan taught himself to read and write and discovered, to his own surprise, that he wasn't a "retard" after all. He also became a Jehovah's Witness and, with the help of the Bible, he has learned how to live peacefully in prison. The last time he had a disciplinary ticket was in 1996. "I can see now how I messed up my whole life. But I've also learned something in all these years inside: how to be a man, how to respect people, how not to take life for granted."

Under the terms of his sentence, though, he will never be able to argue in front of a parole board that he has changed. His most recent - and final - appeal against his sentence was in October 2001. His lawyers argued that to keep him inside any longer was tantamount to warehousing. They said he had proven himself to be a reliable, God-fearing and reformed person.

The judgment handed down at the end of that appeal sits in his legal file. It reads: "It is ordered and adjudged that the petition for habeas corpus be, and hereby is, DISMISSED WITH PREJUDICE."

So what if life continues to mean life? What if this autumn's bill falls? What if Donald Logan, and all the others, remain in their cells to the day they die?

I had been intending to put some of those questions to a man, now aged 70, who knows he will never see the outside of a prison again. Allen Smith. Prisoner 085017. Height: 6ft. Weight: 158lb. Date of birth: December 6 1936. He sent me word that he was keen to talk to me, but warned that he hadn't been feeling well lately and had been spending a lot of time in the prison infirmary. He would telephone me at his lawyer's office.

As I waited for the call, I read his file. Smith had an unhappy relationship with his stepfather, had been in and out of homes, and had a reputation for angry outbursts. He was 16 when, on December 2 1953, he walked up to the house of an old couple he knew well, Robert and Celeste Holton. He said he only intended to steal from the couple but things got out of control. He picked up a deer rifle that happened to be standing by their refrigerator. Afterwards, he said he didn't know what came over him, why he emptied the whole cartridge and killed them both.

He was arrested that day and taken to prison. The lawyer appointed to his case advised him to plead guilty. That way he would be out after 10 years. "Listen to him, son," said the sheriff. "He's telling you right." Smith did as he was advised and on December 19, just 17 days after the murder, he was sentenced to life without parole. The judge said he had no choice. "It is hard on the court and hard on you and hard on everybody, and too hard on the two dead people, but crime does not pay," he said.

Smith spent several years in solitary confinement with hard labour. He writes in his file that soon after the murder he was struck by a deep feeling of remorse. "I had hoped they'd take my own life, and I couldn't understand how I could have done such a thing to such wonderful people, because they really were. It really tore me apart for a long time."

Details from the file are sketchy, but he has clearly suffered other periods of anger and despair, punctuated by moments of hope and happiness. He realised only a few years into his sentence that he would stay in jail for ever. In 1976 he tried to build a better life for himself in prison and he married a woman who had been visiting him. At the time he had hopes he might be given a parole hearing. When it became clear those hopes were groundless, he told the woman he wanted a divorce, to spare her. They still write. Then in 1982 he escaped from prison, and was recaptured after a few days. Futilely, he was given an additional five years on his sentence.

Recently he has become more at ease with himself. He took up courses in Bible studies and literacy. He has also versed himself in the law, and until he became ill would act as a legal adviser to younger prisoners, helping them prepare appeals. Smith writes in his file that he has come to accept that he will die in jail. In an entry a few months ago, he says he is certain he has not long to live. He says religion has proved a comfort to him: "I know I have been forgiven."

I spent most of the day in the lawyer's office waiting for his call. It never came.

Nicole Dupure is at the beginning of the journey and has the experience of despair and reconciliation still in front of her. She's started to think about what has happened to her and why. "I just wanted to grow up too fast, I wanted it all right then."

Her mother and father visit her regularly. "I do my best to hide it when I'm not coping. Especially from my dad. He's 73 and he thinks he will die before I get out." Her child by Blevins is now two years old and has been adopted by Dupure's mother. The little girl came to see Dupure for the first and only time in April. Dupure was shocked because she had assumed her daughter would not remember her, but she did and ran into her arms calling, "Mummy!"

It's time to go and I ask Dupure a couple of last questions. Should the bill fail this autumn, should she remain in jail for ever, could she find any value in life? "I'm going to keep positive," she replies. "I'm not going to try to kill myself or anything. I'm in prison, yes, but it's still a life. I'm just in another world now."

And her regrets?

"I just wish I'd listened to my mum. She laughs when I say that and says, 'It's a little late now, Nic.'"

I stand to leave, walking back past the observation post and into the antechamber with the sliding iron doors. The woman guard steps forward to meet me and shouts, "Closing!" for a second time.