A long time ago I taught summer school on the Navajo Indian Reservation in Arizona. My class was a group of 13-year-olds. Many of the children lived in hogans, small huts made of either mud or wood. They rode horses across the desert and their parents raised sheep which grazed the underbrush.
Their society was uncompetitive and unmaterialistic. These children's world was as different as you can imagine from the New York streets where I had grown up.
But the girls peering out from behind their hair in the photographs I took took exactly like the pictures of my own friends at that age - sullen, self-conscious, subtly challenging. What is it about 13-year-olds? What makes adolescents round the world so difficult?
We have long known that hormones can be held to account, but the latest scientific research shows that teenagers' brains, too, are in a state of change. It is not only emotional immaturity that causes adolescents to over-react and sulk in the corner. Their brains, the scientists say, may also be misreading the sympathetic facial expression on your face.
The good news is that these developing brains are malleable. Teachers can literally help shape them. Good teaching helps young minds develop their capacity to make connections and build emotional strength. That's why it's such a shame that the very distinctive age group of 11 to 14 year olds suffered from systemic neglect for so many years, with pupils being viewed either as overgrown primary children or underdeveloped O-level or GCSE candidates.
Fortunately, things are changing. Not only are teachers and schools becoming more inventive, but for the first time there is a national focus on the particular needs of 11 to 14-year-olds in the round. Key stage 3 may at last be moving on - no longer just a collection of subjects with some controversial tests at the end, but a time to concentrate on how adolescents learn, behave and develop.