The Wisconsin Supreme Court had declined to find a life without possibility of parole sentence unconstitutional in the case of a fourteen year old convicted of first degree murder, a ruling relevant to Colorado because it also has young juveniles serving life without possibility of parole sentences.
The issues are not strictly parallel to those in Colorado, however.
In the Wisconsin case, the trial court had the authority to impose a lesser sentence and declined to do so based on the particular facts, circumstances and mental capacity of the defendant. The crime of conviction also involved personal involvement of the juvenile in the killing.
In Colorado, a juvenile tried as an adult and convicted of first degree murder was automatically given a life without possibility of parole sentence (the death penalty was unavailable) without regard to the facts, circumstances or mental capacity of the defendant, and the judge had no ability to either mitigate the sentence or to make a determination that the juvenile was not eligible to be tried as an adult (a decision vested solely in prosecuting attorneys under Colorado's direct file law at the time). The only role that a judge and jury had in determining the sentence was to determine if the juvenile committed the crime of first degree murder, which in Colorado, includes criminal liability as a participant in a crime that causes a death even if the juvenile was not actually personally involved in the murder itself.
Given the wide deference that judicial determination of facts relevant to sentencing is given on appeal, it was therefore far harder for the juvenile in Wisconsin to make an "as applied" challenge to the constitutionality of a life without possibility of parole sentence than it would be to do so in Colorado. Notably, in death penalty cases, non-discretionary death penalty sessions that do not allow juries to individually weight aggravating and mitigating circumstances where a conviction authorizes the death penalty are not allowed, and there is at least some case law that seems to say that just as "death is different," that "juvenile life without parole" is different.
Thus, the only part of the Wisconsin precedent that is really directly applicable to Colorado as persuasive authority is its finding that the United States Constitution does not categorically prohibit the imposition of a life without possibility of parole sentence upon a fourteen year old for first degree intentional murder.
Specifically distinguishing young juveniles was something that might have been relevant to the "unusual" part of the cruel and unusual punishment analysis because only a small portion of juveniles sentenced to life without possibility of parole sentences are young juveniles and fewer states permit these sentences to be imposed on them, either directly, or because they are not eligible to be tried as adults for that crime. But, the Wisconsin Supreme Court, a conservative leaning court by most accounts after the more conservative candidate narrowly won its most recent highly contested judicial elections, chose not to find this sufficient grounds to find that Wisconsin's statute was categorically unconstitutional.
The Larger 8th Amendment Legal Context
In general, states have very wide discretion to set sentences for criminal offenses committed by adults and juvenile offenders.
This decision comes against a fairly well tested backdrop of precedents on the constitutional boundaries on criminal punishment under the 8th Amendment protection against cruel and unusual punishment and some other constitutional provisions with similar effects.
The federal constitutional issues, although not any other issues raised in the Wisconsin case, could be appealed to the U.S. Supreme Court, which has held unconstitutional a Florida statute that imposed a life without parole sentence in cases that do not involve murder or "crimes against the state" (e.g. treason and espionage) as a violation of the Eighth Amendment protection against cruel and unusual punishment as incorporated to apply against the states under the Fourteenth Amendment to the United States Constitution.
The U.S. Supreme Court has prohibited the use of the death penalty for cases that do not involve murder or "crimes against the state" or for juveniles under the age of eighteen or for the mentally retarded. For example, it held that the death penalty was not permitted for the rape of a child. It is not entirely clear to me if the death penalty is constitutional in cases where proof of a murder is not required by is strongly presumed under the circumstances (e.g. a kidnapping where the victim has not been released alive but not been established to be dead), something that is permitted on the statute books in some states but has never been tested under currently applicable constitutional law to my knowledge. No one is currently on death row for such an offense in the United States, but the justification for such laws is similar to the justification for the death penalty in cases involving "crimes against the state" like espionage.
The U.S. Supreme Court has also held, however, that felony-murder convictions as we as murder convictions based upon personally killing someone or soliciting a murder are eligible for the death penalty (although the role of the person involved is something that a sentencing jury may consider when it decides whether or not to impose a death penalty for a first degree murder), and the U.S. Supreme Court has ceased to make a distinction between young juveniles and older juveniles in eligibility for the death penalty, something that it had done under prior law. The certiorari decisions the U.S. Supreme Court made in its juvenile life without possibility of parole case from Florida had seemed to indicate that at least some justices found a distinction between young juvenile and older juveniles to be meaningful, but it ultimately did not reach that question in that round of cases.
Corporal punishment other than the death penalty, and methods of imposing the death penalty, are at least constitutionally suspect in some cases, particularly if it foreseeably results in, a gratuitous and unnecessary infliction of pain. Similarly, rape as a form of legally authorized punishment by the state is probably unconstitutional. There is also a separate part of the law of cruel and unusual punishment that pertains to the conditions of a sentence of incarceration rather than to the length of the sentence. The most actively litigated controversy in this area is the extent to which long term solitary confinement imposed in the discretion of a prison warden is constitutional. To date, no federal constitutional precedents that I am aware of place clear boundaries on this practice, but it is attracting increasing legislative and judicial attention.
I am not familiar with constitutional precedents on a minimum age at which adult criminal punishments are constitutional. Most states observe some lower bound and exercise discretion to refrain from doing so in a large share of cases where it might have the constitutional authority to do so. But, there is probably some point at which, for example, a life without possibility of parole sentence imposed for a murder committed by a very young juvenile, perhaps a six year old, would be held to be cruel and unusual.
Furthermore, the constitutionality of a non-discretionary life without parole sentence for an adult convicted of first degree murder, whatever its character, has been upheld in hundreds, if not thousands, of cases. Similarly, no serious challenges have had success challenging life sentences where there is a possibility of parole, or long sentences for a term of years, for either juveniles or adults for anything but the most trivial offenses when committed by non-recidivist offenders. I am not aware of any case law that addresses the question of whether a juvenile's criminal record is relevant to the 8th Amendment constitutionality of a very long sentence for a fairly minor crime committed as an adult, in part, because few states do this in practice.
Cases under California's three strikes law have established that a life without possibility of parole sentence, even for petty theft, when committed by an adult who has been convicted of two prior serious (but non-violent) adult felonies, is not an unconstitutional violation of the 8th Amendment's protection against cruel and unusual punishment. Very long sentences, although not life sentences, for minor first time drug possession convictions, have also been upheld as constitutional. The only practical limitations on criminal sanctions for minor offenses are procedural - a right to a jury trial (in cases where a sentence could be to more than six months of incarceration) and of indigent parties to an attorney (if incarceration can be imposed as part of the sentence) must be available if sentences of a certain severity are a possibility upon a conviction. As a general rule, an individual can be arrested and detained briefly consistent with the constitution, even for petty offenses for which incarceration is not available as a sentence upon conviction.
I know of no cases that have found that a sentence to prison for a term of years for one or more offenses that is so long that it amounts to a life without possibility of parole sentence is unconstitutional, although it makes logical sentence that this would be the case in some very long juvenile sentences for offenses other than murder or "crimes against the state" (e.g. a sentence of 120 years in prison without possibility of parole until 90 years have elapsed for a rape committed by a seventeen year old defendant).
A few cases have found unconstitutional criminal statutes that impose a longer sentence than the sentence imposed for an unequivocally lesser crime or an identical crime committed by someone with another constitutionally irrelevant difference in status under the equal protection clause of the 14th Amendment, rather than under any sort of proportionality principal of the 8th Amendment. But, those cases has largely involved idiosyncratic fact patterns. Somewhat meaningful limitations on the size of punitive damage awards in civil cases relative to the amount of compensatory damages awarded have also been imposed as a matter of constitutional law in some circumstances.
Finally, there is a great deal of jurisprudence under the federal criminal code and U.S. Sentencing Guidelines over what sentences within the statutory maximum sentence authorized by a jury are "reasonable" or in contrast, constitute an abuse of discretion by a judge and does not adequately justify a departure from the sentencing guidelines. These cases have invalidated sentencing decisions that are well within the boundaries of statutory maximum penalties for offenses that have been held to be constitutional, but also sometimes invalidate sentences for being too lenient. But, these cases are questions of statutory interpretation rather than constitutional limitations on sentencing.
One of the most controversial parts of the U.S. Sentencing Guidelines jurisprudence is the part that has upheld the constitutionality of judicial consideration of acquitted conduct or uncharged conduct in imposing a sentence less than or equal to the statutorily authorized sentence for the crimes of conviction. So, for example, someone with no prior criminal record convicted of a low level felony larceny count that might ordinarily garner a short sentence of incarceration (perhaps a year in prison) under the U.S. Sentencing Guidelines can constitutionally be given the maximum sentence available for someone with a long criminal record and large dollar value theft under that offense (perhaps twenty years in prison), if the judge believed that the individual also murdered someone, even if the jury acquitted the defendant of that charge.
In principle, the largely unrelated line of cases imposing limits on conduct that can be subject to criminal sanctions at all also imposes constitutional limitations on criminal sentencing, but these limitations are mostly unrelated to the line of cases related to cruel and unusual punishment. Notably, the U.S. Supreme Court has declined, for example, to prohibit criminal defamation statutes in circumstances where civil liability for defamation would be permitted.
Of course, Governors of states that impose juvenile life without possibility of parole sentences, and the President in the case of federal juvenile life without possibility of parole sentences, can commute a life without parole sentence to a lesser sentence or pardon the individual and have them released from incarceration. These decisions are not bound by precedent in any way.
Colorado's Governor Ritter convened a new panel to advise him on juvenile clemency and grant a few instances of clemency based upon its recommendations, but including two juvenile life without possibility of parole sentences in Colorado. The weight of the decision on the other cases currently rests on Governor Hickenlooper's shoulders for the several dozen juveniles serving these sentences in Colorado (no new juvenile life without parole sentences can be imposed in the state; newly sentenced juveniles convicted of first degree murder are eligible for parole after forty years).