The United States Supreme Court is considering a case that could determine whether the execution of teenage killers violates the US constitution.
The debate centres on Christopher Simmons, 17 when he killed
Capital punishment is allowed in 19 US states for those aged 16 and 17, and 73 juvenile murderers are on death row.
Executions for under-16s and the mentally retarded are barred.
The European Union, former President Jimmy Carter and many foreign governments are among those urging an end to the juvenile death penalty.
The case being examined by the Supreme Court judges centres on 17-year-old Christopher Simmons, who was given the death penalty for kidnapping and killing a woman in Missouri in 1993.
His sentence was later overturned on appeal by Missouri's Supreme Court, which argued that a national consensus had developed against capital punishment for offenders under 18.
Missouri's attorney-general appealed to the US Supreme Court, arguing that the death penalty should stand.
The debate throws the spotlight once again on the highly controversial and emotive subject of capital punishment in the US.
Opinion polls suggest a large majority of Americans still favour the death penalty.
'Cruel and unusual'
People with sleeping bags arrived at the Supreme Court in Washington before dawn to try to ensure a seat and by morning a queue of hundreds of people stretched down the street, the AP news agency reported.
Opponents of the death penalty hope the court will rule that the execution of juveniles violates the Eighth Amendment to the Constitution that forbids "cruel and unusual punishment".
But supporters of capital punishment argue age should not determine the gravity of a crime.
"The death penalty is reserved for the worst of the worst. It is not just for adults," Dianne Clements, of victims' rights group Justice For All, told AP.
"It doesn't matter how old the killer is. What matters is that your loved one is gone."
The trend is, in practical terms, already against juvenile executions. In the last 10 years, only Oklahoma, Texas and Virginia have executed people who were under 18 when they committed their crimes.
Just a few other countries, including Saudi Arabia, Pakistan, Iran and China, execute young offenders.
The nine judges have been divided in the past on capital punishment questions, voting 6-3 to bar the execution of mentally retarded offenders.