Normally cooperative and happy preteen children can change seemingly overnight, becoming irritable at the drop of a hat, reacting impulsively and at times, with angry outbursts. What is happening here?
The preteen years are a time of explosive growth, and some of these body changes can cause children to behave in such a way that even they don't understand, much less you. One of the biggest changes occurs in the brain, a process that only recently has been better understood through MRI studies of children's brains.
During the ages between 6 and 12, the nerve pathways in the brain make more and more connections with other nerve cells, increasing nerve signal transmission and making the brain more efficient. But some areas of this new "construction" develop more slowly than others. The pre-frontal cortex of the brain, which is the area of rational thinking, is the last area to develop; in fact, preteens rely more heavily on the amygdala, the "gut reaction" center of the brain, to process emotional information, which explains why they may react more impulsively than adults.
The prefrontal cortex is also home of executive functions such as planning, making priorities, organizing thoughts, and weighing the consequences of actions. This means that the part of the brain that makes your preteen act in a mature and responsible way isn't developed sufficiently enough for him to do so reliably.
Added to the brain changes is a surge of sex hormones released by the adrenal glands. These hormones are very active in the area of the brain's emotional center, attaching to receptors there and influencing the production of "feel good" chemicals such as serotonin. This means that not only will your preteen reach an emotional boiling point more quickly but also that she seeks out experiences to fuel this tinderbox of emotions. This results in a desire to take risks - at precisely the time that the prefrontal cortex is unable to put the brakes on those desires.
Most parents find it useful to learn that there are physiological reasons for their children's behaviors, rather than simply a desire to drive parents crazy. Kids may simply not have much conscious control over their behavior, and while you can't change this, you can help them to make up for what their brain lacks by guiding them through difficult decisions, and providing structure and stability in their environment.