This is an immensely difficult question, involving issues at the core of our criminal justice philosophy. When we punish those who have committed crimes, what is it that we're trying to accomplish? Based on that objective, how do we decide how severely someone should be sentenced? Should that severity be based on the nature of the offense, the defendant's character and history, or some combination of the two?
There are no simple, obvious answers to those questions. Over the centuries, civilized societies have answered them in different ways, and have focused on four major principles. One is retribution--the idea that the punishment should be based on how morally blameworthy the offender is. Another is deterrence--we impose punishments and let the public know about them to dissuade people from committing similar crimes in the future. Another is incapacitation--when someone has demonstrated a risk of harming others, we put him in a place where he's less able to do so, at least for a time. The final one is rehabilitation--providing the offender an opportunity to better himself through education, reflection (think of the origin of the term "penitentiary"), and other experiences associated with a sentence.
These principles are conceptually quite different from one another, and can be in conflict in an individual case. For example, if someone steals out of a desperate need to feed his family, his lawyer may suggest that he's less morally blameworthy than someone who steals for other reasons and should get a less severe sentence (retribution), but the prosecutor could respond that a heavier sentence is necessary to send a message to future thieves (deterrence). Or a defendant in a "cold case," caught and convicted many years after the offense, might claim that because he's turned his life around there's no need to send him to prison (rehabilitation/incapacitation); at the same time, simply setting him free would conflict with the goals of retribution and deterrence.
The tension between these principles is particularly high for defendants who for one reason or another have more difficulty than others in making decisions or appreciating the consequences of their actions. Most criminal law principles are based on the notion of a rational person who fully appreciates the nature of the choices he makes and fully grasps how those choices will affect others--for example, the professional bank robber who takes other people's money because it's easier than getting an honest job, or the white-collar criminal who cooks the books to prop up a company's stock price.
Those people are definitely out there. At the same time, the criminal justice system includes a huge number of people who don't fit that classic model because their cognitive or decisionmaking abilities are seriously impaired. Some of these people are at least partially responsible for the positions they're in, as when the impairments result from drug or alcohol abuse. Others have genetically based mental illnesses or have experienced truly horrific upbringings, neither of which they can plausibly be blamed for. For all such defendants, their cases present the same questions--when they commit serious crimes and we're deciding how severely to punish them, how much attention should we pay to circumstances that may have affected their decisionmaking?
Youth, of course, is one of these factors. No one seriously disputes that in at least some cases young offenders should be treated differently from adults; it's why we have a separate system for juvenile justice. As the Supreme Court pointed out a few years ago, the differences between young people and adults affect all four of the traditional sentencing principles. Most would agree that a juvenile's immaturity and the scientifically demonstrable incompleteness of his brain's development make him at least somewhat less blameworthy than a fully developed adult guilty of the same offense. The same principles make juveniles as a group less rational in their thinking and thus less likely to be deterred from crime by seeing the consequences imposed on others. And because a young offender's character is often not fully formed, he typically has greater potential for true rehabilitation, which if successful reduces the need for long-term incapacitation.
An ideal system might include a Solomonic judge who could accurately measure all relevant circumstances and take them perfectly into account in deciding how they should affect an offender's punishment. Unfortunately, the system is largely based on bright lines, and not only between juveniles and adults. For example, someone who is so mentally ill that he literally can't understand what he's doing can avoid conviction based on the (rarely successful) insanity defense, while someone just on the other side of that line can be convicted and sentenced as a "sane" person despite suffering from grave mental defects. Similarly, in a capital murder case, proof that a defendant suffers from an "intellectual disability" will spare a defendant from execution; in some states, this determination is made based largely on an arguably arbitrary IQ score cutoff (though the Supreme Court recently put some limits on this practice). In all of these situations, "either-or" determinations seem inconsistent with what we know about the vast complexity of human beings' brains, experiences, and abilities.
I can't ultimately say that it's wrong that Jamarion Lawhorn is being tried as an adult. The alleged crime was a grave one and demands serious treatment, and I can't second-guess those who have determined that adult prosecution is the appropriate response. I hope, however, that if Jamarion is convicted, those deciding his ultimate fate can, in the Supreme Court's words, "consider [him] as an individual and [his] case as a unique study in the human failings that sometimes mitigate, sometimes magnify, the crime and the punishment to ensue." That's the path to justice in this awful case.