You might be talking about the Stanford v. Kentucky (1989), where the U.S Supreme Court ruled that offenders at least 16 years old at the time of the crime can be punished with death penalty. Doing so would not constitute cruel and unusual punishment. That means 16-year olds who committed heinous crimes can be put to death. The ruling was overturned with the Roper v. Simmons in 2005, which forbade capital punishment for juveniles.