18 and 19 and 20 year-olds are not considered responsible enough decision makers to drink legally, yet they can be held fully responsible for their crimes and sentenced to the ultimate, irreversible punishment of death.
Yokamon Hearn was 19 years old when he and 3 other youths set out to steal a car. They ended up shooting and killing Frank Meziere, a 23-year-old stockbroker. All four defendants were charged with capital murder, but the other three plead guilty and received deals. One got life imprisonment, the other two got ten years for aggravated robbery.
Yokamon Hearn was a teenager at the time of his crime, but not a juvenile. Article 37 of the Convention on the Rights of Child lays out the international standard for not executing juvenile offenders, defined as those who were under 18 at the time of the crime. (The U.S. is the only country except for Somalia that has not ratified this treaty.)