A glance at practice in other nations supported the United States Supreme Court's decision (6-3) that the Constitution does not permit a person under the age of 18 years at the time of his offending to be sentenced to life imprisonment without parole for a nonhomicide offence: Graham v Florida  USSC No 08-7142, 17 May 2010.
The Eighth Amendment, "Excessive bail shall not be required, nor excessive fines imposed, nor cruel and unusual punishments inflicted", is the governing provision. Kennedy J, delivering the opinion of the Court, observed:
"To determine whether a punishment is cruel and unusual, courts must look beyond historical conceptions to " 'the evolving standards of decency that mark the progress of a maturing society.' " Estelle v. Gamble, 429 U. S. 97, 102 (1976) (quoting Trop v. Dulles, 356 U. S. 86, 101 (1958) (plurality opinion)). "This is because '[t]he standard of extreme cruelty is not merely descriptive, but necessarily embodies a moral judgment. The standard itself remains the same, but its applicability must change as the basic mores of society change.' " Kennedy v. Louisiana, 554 U. S. ___, ___ (2008) (slip op., at 8) (quoting Furman v. Georgia, 408 U. S. 238, 382 (1972) (Burger, C. J., dissenting))."
The criterion is proportionality, and the Court has adopted a classification of cases: there are firstly, cases where all the circumstances are considered. For example,
"...closely divided, the Court rejected a challenge to a sentence of 25 years to life for the theft of a few golf clubs under California's so-called three-strikes recidivist sentencing scheme. Ewing v. California, 538 U. S. 11 (2003); see also Lockyer v. Andrade, 538 U. S. 63 (2003)."
These startling but acceptable punishments have been upheld in a decision process where the offender (appellant) must show, as a threshold, gross disproportionality between the sentence and the crime before the appellate court will proceed to compare his sentence with those imposed in the same jurisdiction and in other jurisdictions. The sentence is only cruel and unusual if this comparison confirms the initial perception of gross disproportionality.
The present case was not in this category.
The second category in the classification of cases is where rules are applied to define Eighth Amendment standards. Until now, these cases had concerned the death penalty. There were two subsets of cases in this second category: those considering the nature of the offence, and those considering the characteristics of the offender. So, the death penalty cannot be applied where the crime against an individual is not a homicide. Nor can it be applied where the crime was committed when the offender was under 18 years old. Nor where his intellectual functioning was in a low range.
Since the present case was not in the first category, the threshold approach was not applicable. This case sought an addition to the subsets in the second category so that it would apply to all those who were under 18 years old at the time of their offending.
The advantage of a case being in the second category is that without a threshold having to be met the Court would look at the national legislation and sentencing practice to see if there is consensus indicating society's standards, then the Court would exercise its own independent judgment to decide whether the punishment violates the Eighth Amendment in the light of precedent, text, history, meaning and purpose.
The Court looked at national sentencing practice and concluded
"The sentencing practice now under consideration is exceedingly rare. And "it is fair to say that a national consensus has developed against it." Atkins v. Virginia, 536 U. S. 304 (2002)16, at 316."
"The judicial exercise of independent judgment requires consideration of the culpability of the offenders at issue in light of their crimes and characteristics, along with the severity of the punishment in question."
The Court's precedents regarding the characteristics of juvenile offenders, their vulnerabilities and their prospects for reform on maturity, the need for a line to be drawn between murderers and other offenders, and the severity of a sentence of life imprisonment without parole especially for juveniles, the absence of any penological justification (retribution, deterrence, incapacitation, rehabilitation) for such a sentence, led to the conclusion that for a juvenile offender who did not commit homicide the Eighth Amendment forbids the sentence of life without parole.
So, why the age of 18 as the dividing line?
The alternative would be a case-by-case approach to assessing an offender's responsibility, but the Court rejected that because a sentencing court could be easily influenced, by the depravity of offending, to underestimate the offender's chance of reform, and also because juveniles do not necessarily communicate adequately with their counsel. All juveniles should be given the opportunity to demonstrate maturity, to show remorse, and to achieve rehabilitation.
Looking beyond the borders of the USA, the Court noted that the sentencing policy hitherto adhered to is rejected the world over.
"This observation does not control our decision. The judgments of other nations and the international community are not dispositive as to the meaning of the Eighth Amendment. But " '[t]he climate of international opinion concerning the acceptability of a particular punishment' " is also " 'not irrelevant.' " Enmund, 458 U. S., at 796, n. 22."
"We also note, as petitioner and his amici emphasize, that Article 37(a) of the United Nations Convention on the Rights of the Child, Nov. 20, 1989, 1577 U. N. T. S. 3 (entered into force Sept. 2, 1990), ratified by every nation except the United States and Somalia, prohibits the imposition of "life imprisonment without possibility of release . . . for offences committed by persons below eighteen years of age.""