Up until the 20th century, "adolescence" was not a meaningful concept to our cultural ancestors. Psychologists are now seeing the development of another new stage in life for American youth: Emerging Adulthood.

During the ... emerging adulthood ... young men and women are more self-focused than at any other time of life, less certain about the future and yet also more optimistic, no matter what their economic background. This is where the “sense of possibilities” comes in ... they have not yet tempered their ideal istic visions of what awaits. “The dreary, dead-end jobs, the bitter divorces, the disappointing and disrespectful children . . . none of them imagine that this is what the future holds for them,” he wrote. Ask them if they agree with the statement “I am very sure that someday I will get to where I want to be in life,” and 96 percent of them will say yes. But despite elements that are exciting, even exhilarating, about being this age, there is a downside, too: dread, frustration, uncertainty, a sense of not quite understanding the rules of the game ... .

Some scientists would argue that this ambivalence reflects what is going on in the brain, which is also both grown-up and not-quite-grown-up. Neuroscientists once thought the brain stops growing shortly after puberty, but now they know it keeps maturing well into the 20s. This new understanding comes largely from a longitudinal study of brain development sponsored by the National Institute of Mental Health, which started following nearly 5,000 children at ages 3 to 16 (the average age at enrollment was about 10). The scientists found the children’s brains were not fully mature until at least 25. “In retrospect I wouldn’t call it shocking, but it was at the time,” Jay Giedd, the director of the study, told me. “The only people who got this right were the car-rental companies.”