WASHINGTON (CNN) — The Supreme Court struggled in somber tones Tuesday when confronting one of its toughest criminal sentencing questions: whether two men convicted of killings committed when they were 14 years old deserve life in prison without the possibility of parole.

The justices seemed to find little agreement on how young is too young in these rare instances, and whether it would be cruel and unusual punishment to forgo the chance that these now-adult inmates may someday be rehabilitated.

“What’s the definition of a child” asked Justice Samuel Alito, trying to draw a constitutionally acceptable line on mandatory sentences for underage murderers.

“Some issues may justify different treatment,” said Chief Justice John Roberts.

The high court in 2005 banned the death penalty for those under 18 who commit aggravated murder. Then, five years later, the justices said juveniles found guilty of non-homicides could not receive life without parole.

Now the spotlight turns on the youngest of killers and the question of whether a national consensus has developed to treat them differently regarding a lifetime of incarceration.

The separate appeals involve an Alabama boy who, with an accomplice, robbed a neighbor and then beat the man to death and set his house on fire; and an Arkansas youth who was part of a group of fellow teens who robbed a video store, where the clerk was blasted to death with a shotgun.

Both were tried and convicted as adults, receiving the minimum sentences allowed under state law for felony capital murder.

The cases are Miller v. Alabama (10-9646) and Jackson v. Hobbs (10-9647). Rulings are expected within three months.