At what age is a person sufficiently mature to receive a death sentence or a sentence of life in prison without parole for committing murder? Ask 10 people and you may get 10 different answers.
Ask the U.S. Constitution and you will get no specific answer.
Here is what the Fifth Amendment says:
“No person shall be … deprived of life, liberty, or property, without due process of law … .”
With due process of law, therefore, a person may indeed be executed or imprisoned – including permanently – for a given crime.