The case you are referring to is Wilkins v. Missouri, a consolidated Supreme Court case from 1989.
Wilkins v. Missouri was consolidated with Stanford v. Kentucky and considered together by the United States Supreme Court. The central question of the consolidated 1989 case was whether it is a violation of the Eighth Amendment Prohibition of Cruel and Unusual Punishment to sentence capital offender to death when said capital offender was under the age of 18 at the time of the committed capital crime.
The Wilkins v. Missouri case involved a minor, Heath Wilkins, who, at the age of 16 years and 6 months, stabbed and murdered a convenience store worker in the process of robbing the store. Due to the viciousness of Wilkins actions and his previous juvenile crimes, Wilkins was tried as an adult and sentenced to death.
The Supreme Court, in a 5 to 4 decision in the consolidated case, ruled that sentencing a minor aged 16 or 17 is not unconstitutional. The Court's decision rested primarily on the notion that there was no national consensus on whether it is appropriate to sentence such young offenders to death. The Court cited numerous conflicting state laws, public opinion and studies as evidence of this lack of consensus. Thus, the Court held that whether it is appropriate to sentence a 16 or 17 year old to death is a subject best left to the states to determine, at least at the time of the decision.