Constitutional Connection

The 8th amendment of the U.S. Constitution states, "Excessive bail shall not be required, no excessive fines imposed, nor cruel and unusual punishments inflicted." This amendment forbids some punishment entirely like for example sentencing juvenile death penalty is against the law. Mostly the 8th amendment prevents cruel and unusual punishment inflicted compared to the crime or compared to the competence of the perpetrator. In the Constitution it states what I explained above but does not explain what is considered cruel and unusual punishment, so is giving juveniles lifetime imprisonment cruel or not depending on the severeness of the committed crime? This is another reason that this issue is controversial.