Psychology Today talks with psychologist Robert Epstein about his book, The Case Against Adolescence:
In every mammalian species, immediately upon reaching puberty, animals function as adults, often having offspring. We call our offspring "children" well past puberty. The trend started a hundred years ago and now extends childhood well into the 20s. The age at which Americans reach adulthood is increasing -- 30 is the new 20 -- and most Americans now believe a person isn't an adult until age 26.
The whole culture collaborates in artificially extending childhood, primarily through the school system and restrictions on labor. The two systems evolved together in the late 19th-century; the advocates of compulsory-education laws also pushed for child-labor laws, restricting the ways young people could work, in part to protect them from the abuses of the new factories. The juvenile justice system came into being at the same time. All of these systems isolate teens from adults, often in problematic ways.
Epstein says the infantilization of adolescents creates a lot of conflict and isolation on both sides of the divide. Over at Marginal Revolution, economist Tyler Cowen adds:
The problem, of course, is that a contemporary wise and moderate 33 year old is looking to climb the career ladder, find a mate, or raise his babies. He doesn't have a great desire to educate unruly fifteen year olds and indeed he can insulate himself from them almost completely. He doesn't need a teenager to carry his net on the elephant hunt. Efficient capitalist production and rising wage rates lead to an increased sorting by age and the moral education of teens takes a hit.
You can read the first chapter of the book at The Radical Academy.
Update: Bryan writes to recommend Neil Postman's The Disappearance of Childhood, saying that "Postman argues that the idea of childhood is a cultural phenomena that comes and goes through the ages". (thx, bryan)