And, perhaps, a crime.
Steinberg, a Temple University psychology professor, helped draft an American Psychological Association brief for a 2005 case in which the U.S. Supreme Court outlawed the death penalty for crimes committed before age 18.
That ruling relies on the most recent research on the adolescent brain, which indicates the juvenile brain is still maturing in the teen years and reasoning and judgment are developing well into the early to mid 20s. It is often cited as state lawmakers consider scaling back punitive juvenile justice laws passed during the 1990s.
"As any parent knows," wrote Justice Anthony Kennedy for the 5-4 majority, youths are more likely to show "a lack of maturity and an underdeveloped sense of responsibility" than adults. "... These qualities often result in impetuous and ill-considered actions and decisions."
He also noted that "juveniles are more vulnerable or susceptible to negative influences and outside pressures, including peer pressure," causing them to have less control.
Some child advocates have pointed to the Supreme Court decision and the research as evidence that teens - even those accused of serious crimes - should not be regarded in the same way as adults in the criminal justice system.
Dr. David Fassler, a psychiatry professor at the University of Vermont College of Medicine who has testified before legislative committees on brain development, says the research doesn't absolve teens but offers some explanation for their behavior.
"It doesn't mean adolescents can't make a rational decision or appreciate the difference between right and wrong," he said. "It does mean, particularly when confronted with stressful or emotional decisions, they are more likely to act impulsively, on instinct, without fully understanding or analyzing the consequences of their actions."
Experts say that even at ages 16 and 17, when compared to adults, juveniles on average are more:
Violence toward others also tends to peak in adolescent years, says psychiatrist Dr. Peter Ash of Emory University in Georgia. It's mostly likely to start around age 16, and people who haven't committed a violent crime by age 19 only rarely start doing it later, he said.
The good news here, he said, is that a violent adolescent doesn't necessarily become a violent adult. Some two-thirds to three-quarters of violent youth grow out of it, he said. "They get more self-controlled."
Some of the changes found in behavioral studies are paralleled by changes in the brain itself as youths become adults.
In fact, in the past few years, Steinberg said, brain scans have given biological backing to commonsense notions about teen behavior, like their impulsiveness and vulnerability to peer pressure.
Consider the lobes at the front of the brain. The nerve circuitry here ties together inputs from other parts of the brain, said Dr. Jay Giedd of the National Institute of Mental Health.
This circuitry weighs how much priority to give incoming messages like "Do this now" versus "Wait! What about the consequences?" In short, the frontal lobes are key for making good decisions and controlling impulses.
Brain scans show that the frontal lobes don't mature until age 25, and their connections to other parts of the brain continue to improve to at least that age, Giedd said.
The inexplicable behavior and poor judgments teens are known for almost always happen when teens are feeling high emotion or intense peer pressure, conditions that overwhelm the still-maturing circuitry in the front part of brain, Giedd said.
As Steinberg sees it, a teenager's brain has a well-developed accelerator but only a partly developed brake.
By around 15 or 16, the parts of the brain that arouse a teen emotionally and make him pay attention to peer pressure and the rewards of action - the gas pedal - are probably all set. But the parts related to controlling impulses, long-term thinking and resistance to peer pressure - the brake, mostly in the frontal lobes - are still developing.
"It's not like we go from becoming all accelerator to all brake," Steinberg said. "It's that we go from being heavy-foot-on-the-accelerator to being better able to manage the whole car."
Giedd emphasized that scientists can't yet scan an individual's brain and draw conclusions about his maturity. Brain scans do show group differences between adult and teen brains, he said, "but whether or not that should matter (in the courtroom) is the part that needs to be decided more by the judicial system than the neuroscientist."
In any case, experts say, there's nothing particularly magic about the age 18 as a standard dividing line between juveniles and adults in the courtroom. Steinberg noted different mental capabilities mature at different rates, but added it appears age 18 is good enough to be justified scientifically.
Steinberg said he thinks courts should be able to punish some 16- or 17-year-olds as adults - generally repeat violent offenders who've resisted rehabilitation and could endanger other youth in the juvenile system. "I don't think there are a lot of these kids," he said.
For the rest, he thinks it makes sense to try rehabilitating young offenders in the juvenile justice system.
Ash said that to decide whom to treat as an adult, courts need some kind of guideline that combines the defendant's age with the crime he's accused of. That should leave room for individual assessments, he said.
Most experts also conclude that rehabilitation works better for juveniles than for adult offenders. And just as parents know how irrational juveniles can be, Ash said, they also know that rehabilitation is a key goal in punishing them.
"What we really want," he said, "is to turn delinquent kids into good adults."