Written by Bradford Colbert, Professor at Mitchell Hamline, and Alexandra Baker Kroeger, Certified Student Attorney.
Adulthood is a social construct. For that matter, so is childhood. But like all social constructs, they have real consequences. They determine who is legally responsible for their actions and who is not, what roles people are allowed to assume in society, how people view each other, and how they view themselves. But even in the realms where it should be easiest to define the difference—law, physical development—adulthood defies simplicity.
The idea that children have a special status different from adults has been widely accepted for centuries. However, the definition of childhood—who is classified as a child, and what emotional, intellectual, and moral properties a child is assumed to possess—has changed over time in response to societal changes. Similarly, the distinct characteristics of each life-stage—childhood, adolescence and adulthood—have also varied over time and across cultures.
When does a juvenile legally become an adult? The answer varies, depending on the issues at stake. A person can consent to having sex at sixteen and vote at eighteen, but will not be able to drink until he or she turns twenty-one. And in Minnesota, a person as young as fourteen can be certified as an adult in various types of crimes.
In the criminal justice system, the issue of when a juvenile becomes an adult is, quite literally, a matter of life or death because the United States Supreme Court has held that the Constitution prohibits the imposition of capital punishment on a juvenile. But despite the enormous consequences, the Supreme Court has spent precious little time defining what it means to be a juvenile. Instead, the Court has simply accepted the conventional wisdom that a person is considered an adult on her eighteenth birthday: “The age of 18 is the point where society draws the line for many purposes between childhood and adulthood. It is, we conclude, the age at which the line for death eligibility ought to rest.”
But there isn’t any rational or scientific basis for drawing the line at eighteen. Indeed, recent scientific research proves that brain maturation actually occurs from ages ten to twenty-five. This is the same brain research the United States Supreme Court has used to adopt legal principles that both protect, and harm, adolescents.
Drawing the line at eighteen is, quite simply, arbitrary. And arbitrarily drawn lines, almost by definition, violate the Equal Protection Clause of the Fourteenth Amendment which provides that no state shall deny to any person within its jurisdiction “the equal protection of the laws.” The essence of the equal protection clause is to ensure that the government treat similarly situated individuals similarly and not arbitrarily differentiate between similarly situated individuals.
Two people born minutes, even seconds apart, would be similarly situated, but because of the arbitrary drawn line at age eighteen would be treated quite differently by the criminal justice system. Rather than succumb individuals to matter-of-life questions based on a birth date, courts should take an individualized approach to assess the mental age and culpability of the individual.
 Julie Beck, When Are You Really an Adult?, The Atlantic (Jan. 5, 2016), https://www.theatlantic.com/health/archive/2016/01/when-are-you-really-an-adult/422487/
 Minn. Stat. § 260B.125.
 Roper v. Simmons, 543 U.S. 551, 568 (2005).
 Id. at 574.
 What Are the Implications of Adolescent Brain Development for Juvenile Justice?, Coalition for Juvenile Justice 1 (2006), http://www.juvjustice.org/sites/default/files/resource-files/resource_134.pdf.
 See Roper, 543 U.S. at 569 (referencing Johnson v. Texas, 509 U.S. 350, 367 (1993)).
 Stanford v. Kentucky, 492 U.S. 361, 394–96 (1989) (Brennan, J. dissenting) (citing Brief for American Society for Adolescent Psychiatry et al. as Amici Curiae 4 (citing social scientific studies)).