How Long Does Adolescence Last?

Sociologist Michael Kimmel came to campus last semester and an interesting point he made was that adolescence is a modern concept. Traditionally, you hit an age where you are capable of reproduction and, boom, you move from childhood into adulthood instantly. But since Freud's time, only the last century, we've had a different view in which there is a transitional period. It reminds me of that sentence from the opening of "The Gods Must Be Crazy," "...and their children are sentenced to twelve years of scholling just to learn how to survive in this strange world."

So, how long does adolescence last? Has it been expanding? Is it cultural? When is someone an independent adult? Is that a single concept or can you be an adult, that is, be post adolescent, and not be independent?