Jensen has been spreading this message with the zeal of an evangelist. A specialist in age-specific brain injury with a particular focus on epilepsy in newborns, she began exploring adolescent neurology when her two teenage sons baffled her with their bizarre behavior.

Doctors, developmental psychologists, and educators say the brain development message gains urgency as more is learned about the teen brain, a neglected area of research in Jensen’s view. The very qualities that make learning easier in youth also make habits like smoking or drinking more deeply embedded, for example. Then there are differences in brain chemistry that mean antidepressants may act differently on young brains, notably increasing suicidal thoughts in enough cases to warrant a warning from government regulators. While youth is a time of resilience in many ways, it’s a myth that people can make up for damage later when it comes to the brain, Jensen said.

“We all know what the frontal lobe does,’’ said Dr. Frances Jensen, a neurologist at Harvard Medical School and Children’s Hospital Boston. “It’s insight, judgment, inhibition, self-awareness, cause and effect, acknowledgment of cause and effect. And big surprise: It’s not done in your teen years. Hence [teens’] impulsiveness, their unpredictable behavior, their lack of ability to acknowledge and see cause and effect, despite the fact they are getting 800s on their SATs and can be cognitively highly functional and memorize at a much more impressive rate than we as adults do later.’’

Of all the organs in our bodies, the brain takes the longest to develop. Frontal lobes — the seat of judgment — are the last pieces to be fully connected to the parts of the brain that sense danger or solve calculus problems. A growing body of neuroscientific evidence places full brain maturity at about age 25, well past the point when young people begin to drive, drink, vote, or go off to war.

Smart kids doing stupid things: It’s the teen brain paradox. Extraordinarily quick to learn and rapidly reaching fluency in abstract thought, teens still make bonehead decisions, perhaps more so when routines relax in summer. But that’s because they’re operating with brains that are still a work in progress.

“I know some people who are heavy drinkers, and they’ve actually told me they feel their memory is going. They drink and then they black out, more and more,’’ said Davis, a 20-year-old Lexington High graduate who attends Occidental College in Los Angeles. “They don’t change their behavior. I don’t think it’s addiction. I guess that gets into judgment.’’

Adam Davis says one of his brightest friends makes the most ridiculous mistakes. For all his smarts, he’ll cross the street without looking.

“It was like living with this alien species,’’ she recalled. “There was so much I didn’t get.’’

The answer to the paradox is gray and white, neurologically speaking. Gray matter includes brain cells that from birth begin to connect to one another, forming synapses in the small space between two nerve cells where electrical signals travel via chemical transmitters. These connections made during development are also what we use to learn and remember. In order to make new connections between different brain cell groups, cells on both sides have to be “on,’’ or in a state called excitation. Its opposite is inhibition, and in adults the two are balanced. But in the brains of babies, children, and to a lesser extent teenagers, excitation is higher. They’re juiced for connections, Jensen said, making learning almost effortless for a child.

White matter lies below the synapse-making gray matter. Its job is to conduct the signals sent between cells as fast as possible via a fatty substance called myelin. Myelin is nature’s insulation, but it builds slowly, into mid-adulthood.

The brainstem, which controls breathing, for example, has its synapses connected and its myelin insulation in place at birth. By mid-childhood, other parts of the brain are connected and myelinated, too, but those frontal lobes are the last to be completed. The impressive learning ability is there, thanks to levels of excitation higher than in adult brains, but the myelination that would bring judgment into play before crossing the street or deciding to binge drink again isn’t all there yet. Teens just can’t access their frontal lobes as quickly as adults.

Drinking and drugs act potently on the teenage brain, neuroscientists now believe, because addiction itself is a form of learning that banks on better memory formation.

“Teenagers get more robustly addicted and for a longer time,’’ Jensen said. “Things like pot have a longer effect on these teen brains because they have more machinery for those molecules to be connected.’’

When Jensen speaks to high school students, often the day after lecturing to their parents, she gets their attention when she tells them the marijuana they smoke over the weekend will still impair them on a test they take the following Thursday. She firmly believes in appealing to the intellectual side of her young audiences, so she presents annotated slides and cross-sectional views of the brain, just as she would for a scientific meeting.