Hector Manuel Castillo drove the car from which his passenger, Alberto Valles, shot and fatally wounded the occupant of a rival gang member’s vehicle. Both were convicted of first-degree extreme indifference murder.

Castillo, who was 18 years and six months old at the time of the killing, received a sentence of life in prison without parole. Valles, who was two days short of his eighteenth birthday, will be eligible for parole after 40 years.

Despite an age difference of only six months, the Court of Appeals has upheld Castillo’s sentence as constitutional, given, it said, that policymakers and society have chosen to draw the line on maturity and responsibility at 18.

“The line may not be anchored in neuroscientific differences between juveniles and young adults, but that does not make it arbitrary or unreasonable,” wrote Judge Jaclyn Casey Brown in the court’s Feb. 17 opinion.

Stan Garnett, the former district attorney for Boulder County, said there is a high threshold to prevail on the argument that Castillo put forward: that two people are in the same position and should receive the same consequence.

“The court system and our laws are all about drawing distinctions and drawing lines. And one of the brightest lines is between being a juvenile and adult,” he said.

Initially, Castillo and Valles both received mandatory minimum life sentences without the possibility of parole for the Nov. 30, 2005 shooting of Richard Scobee. But in 2013, the Court of Appeals ordered that Valles be parole-eligible after 40 years, pursuant to the U.S. Supreme Court’s decision the prior year in Miller v. Alabama. In a 5-4 ruling, the justices held that the Eighth Amendment’s prohibition on cruel and unusual punishment forbids lifetime sentences without parole for juveniles who commit homicide.

Castillo, representing himself, then asked an Arapahoe County District Court judge for relief unsuccessfully. He turned to the Court of Appeals, pointing out that although he and Valles were both held responsible for Scobee’s murder, only Valles was able to benefit from Miller by a matter of days.

“Certainly this raises a question as to whether anyone believes that the morning following the day of the shooting death,” he wrote, “that (Valles) woke up a changed man, i.e., he transitioned from a juvenile to an adult? No, a better approach is the individual one, without any clear bright-line rule demarcating when an individual transitions to adulthood.”

Castillo also called it “illogical and frankly inconsistent” that he received a longer sentence than Valles, the actual shooter, because of a six-month difference in age.

In an uncommon move, the Court of Appeals appointed an attorney for Castillo, who further contended that Castillo’s sentence violated his right to equal protection under the law by treating differently two defendants who allegedly had similar levels of maturity, impulse control and wisdom.

“At the time of the offense, Mr. Castillo was essentially a juvenile in all but name,” wrote Castillo’s appointed lawyer, Robin M. Lerg. “Other courts are beginning to recognize that science and find that there are no meaningful developmental differences between the brain of a 17-year-old and the brain of an 18-year-old.”

Courts have accepted that children are less culpable for crimes than adults and possess a greater capacity to rehabilitate themselves. Last year, the Colorado Supreme Court struck down mandatory lifetime registration for juveniles on the sex offender registry as a violation of the Eighth Amendment, citing the science behind the psychological development of children.

On the opposite side, the Colorado Attorney General’s Office defended the mandatory lifetime sentence for Castillo, arguing it was reasonable to impose different sentences before and after a person’s eighteenth birthday.

“Defendant has no constitutional right to an equal sentence as his co-defendant. And while he may wish that the General Assembly would have classified him as a juvenile, the legislators had no obligation to do so,” wrote Assistant Attorney General Patrick A. Withers.

The three-judge appellate panel reviewing Castillo’s case determined that Valles and Castillo were, in fact, similarly situated at the time of sentencing due to their closeness in age. However, Castillo needed to prove that differential treatment for 18-year-old offenders was unreasonable, with no rational objective.

The panel believed Castillo failed to make that case.

“Given the privileges and responsibilities associated with adulthood, there are real differences between seventeen-year-olds and eighteen-year-olds. The legislature’s recognition of these differences in establishing criminal penalties furthers a legitimate state interest,” wrote Brown.

Castillo’s attorney declined to say whether Castillo would pursue an appeal.

Garnett believed there is a viable argument for Gov. Jared Polis to reduce Castillo’s sentence. Polis stepped in last year to lower the 110-year minimum sentence of Rogel Aguilera-Mederos, a truck driver whose brakes malfunctioned on Interstate 70 and resulted in the deaths of four people. At the time, Polis told Aguilera-Mederos that the trucker deserved clemency because he had received “more than a life sentence, for a tragic but unintentional act.”

“I think the real solution is to move away from mandatory sentencing an put back some range of discretion the courts can impose, just because every crime is different. The people are different,” Garnett added.

The case is People v. Castillo.