The highest court in the US has said it will consider the legality of executing people who are under the age of 18 when they committed their crimes.
The ruling on the Missouri case will have consequences for other states.
The Supreme Court said it would rule whether such action violated the constitution's ban on "cruel and unusual punishment".
The hearing centres on the case of a man from the state of Missouri who was 17 when he killed a woman in 1993.
Missouri's state supreme court has said execution would be unconstitutional.
It said it made the ruling because the offender was under the age of 18 when he committed the crime.
He was found guilty of murder after robbing a woman and throwing her off a railway bridge.
Now the nine justices on the federal Supreme Court will decide whether the execution should go ahead.
By making the ruling they will also decide whether it is legal for US states to execute those under 18 when they offended.
The human rights group, Amnesty International, has said the US has executed 13 of what it calls "child offenders" since 1998.
It said that was two-thirds of the total worldwide.
The BBC's Adam Brookes in Washington said that opponents of the death penalty seem to have been gaining ground recently.
In 2002, the Supreme Court banned the execution of those deemed "mentally retarded".
Last year in Illinois, the outgoing governor suddenly commuted the sentences of 170 death row inmates; he said the capital punishment system was flawed and unfair.
And in New York, lawyers are demanding higher standards of proof for death penalty convictions.
A Supreme Court decision against the execution of those aged under 18 will continue to roll back the scope and frequency of executions in America, our correspondent says.
A ruling is expected in the summer.