(CNN)For the first time in nearly 70 years, the United States will execute a man for a crime he committed as a teenager.
Christopher Vialva, 40, is set to receive the death penalty on Thursday at a federal prison in Terre Haute, Indiana. He was convicted 20 years ago for the 1999 murders of two youth ministers in Fort Hood, Texas. At the time of the crime, he was 19 years old.
The crime was egregious, his attorney and other advocates agree.
But given decades of research since then on adolescent brain development, the announcement from the Justice Department raises a key question: Should a person be given the ultimate punishment for a crime they committed in their youth?
"Despite the very, very heinous nature of the crime that Christopher has been convicted for, it's my position that based on the science, his brain was not the brain of a fully fledged adult," Jason Chein, a professor of psychology at Temple University and author of a recent op-ed on Vialva's sentence, told CNN. "And that leads me to the conclusion that the punishment of taking one's life is too severe."
CNN reached out to the Department of Justice for comment, but has not heard back.
Adolescent brains are not fully mature, research shows
The Justice Department announced on July 31 that it planned to execute Vialva, along with William LeCroy, who was executed on Tuesday. It will be the seventh federal execution since the Justice Department ended a 17-year hiatus on the practice in July.
Vialva was tried for his role, along with a group of teenagers, in the 1999 carjacking and killing of Todd and Stacie Bagley in Fort Hood, Texas, according to a statement from the Justice Department.
A federal court found that Vialva shot the couple, while co-defendant Brandon Bernard, who was 18 at the time, set the car on fire to destroy evidence. Both were found guilty and sentenced to die.
The crime Vialva was convicted of was "vile" and there's no question that he should be held accountable, Chein said. But research that has emerged since his trial has shown that the brain of someone in their late teens and early twenties is not yet fully mature.
"The evidence tells us that an individual at this age has the potential for change," Chein said. "Their personality and their behaviors are not fixed the way we might think they ultimately become in a more fully matured adult."
Studies show that teenagers are more likely to take serious risks, especially when they are with their peers. They're also less able to control their impulses in stressful or heated situations.
Brain imaging studies show that in adolescents, the region of the brain implicated in decision-making and risk assessment is still developmentally immature. At the same time, the part of the brain that plays a role in regulating pleasure and reward is relatively more active than in other periods of development.
That combination, scientists say, can make for dangerous outcomes.
There's no hard cutoff for when a person matures
The Supreme Court has already acknowledged that young age should be factored into sentencing decisions, advocates argue.
In 2005, the Court ruled that executing a minor constituted as "cruel and unusual punishment" and outlawed the juvenile death penalty. In 2012, it held that imposing a mandatory life sentence without the possibility of parole for juvenile offenders was unconstitutionally disproportionate.
By that logic, the federal government should not seek the death penalty for a person who was convicted for a crime while their brain was still developmentally immature, said Henderson Hill, senior counsel at the American Civil Liberties Union (ACLU).
Vialva, however, is not considered a juvenile because he was 19 at the time of his crime.
"It just flies in the face of good science, what we know about the brain development," Hill told CNN. "... It's just horrible to think that this kind of final judgment can be passed on a 19-year-old."
The way the legal system functions does not take into account the nuances around brain development, Chein said. The process is not monolithic, meaning that different systems in the brain develop at different rates and the point at which a person can developmentally be considered an adult varies from person to person.
"The idea that there's this bright line, this age that you suddenly cross into maturity as you go from 17 to 18, is undermined very much by the evidence from developmental science," Chein said. "There's no moment at which you cross this line, and now you're an adult."
That complicated reality is already evident in how the US treats adults, advocates say.
People can vote and enlist in the military at age 18. But they can't purchase alcohol or tobacco until age 21. Auto rental and insurance companies charge higher rates for individuals under 25 because they're considered to be higher risk drivers.
"He would not have been old enough to buy a pack of cigarettes or alcohol," Hill said. "But he's old enough to be executed. That cannot make any sense."
People have the potential to change as they age
Because a person's brain continues to develop well into their mid-20s, experts say that who a person is at 19 or 20 is not necessarily an indication of the person they'll be at 30 or 40.
Vialva's attorney Susan Otto argues that's the case for her client, who she said has matured significantly since she first met him in 2003. An evaluation of Vialva's brain at the time showed that when he was 19, he had "the developmental acumen of a 16 year old," Otto said.
She pointed to his embrace of the Messianic faith and his remorse for the crime as evidence of his growth as a person.
"I cannot say that he was a bad person when I met him who has somehow turned out well," she told CNN. "I can tell you he was a boy when I met him."
Despite what the research now shows, Otto said that Vialva's intellectual maturity, along with other issues of abuse, mental illness and racism that he experienced growing up, were not a part of his original defense. An examination conducted by a psychologist also found "compelling evidence" that Vialva likely suffers from organic brain damage and/or bipolar disorder, according to a news release from the ACLU.
Because of what she called "deficiencies with his legal representation," she argues that it is unjust for Vialva to be put to death.
"The question is not whether Christopher should be held accountable for the harm he caused — no one disputes that," she said in a statement released by the ACLU.
"It is whether we can truly call it justice to kill Christopher for a mistake he made as a teenager when he did not get the help or defense everyone deserves."