Nature has a feature by Kendall Powell on how teenage brain's work (see also my 7/07 post). Abstracting from that review:
An NIMH research team, led by Jay Giedd, has made a movie of normal brain changes from ages 5 to 20. It reveals that the grey matter thickens in childhood but then thins in a wave that begins at the back of the brain and reaches the front by early adulthood (see movie, below). The process completes itself sooner in girls than in boys. This corresponds to a long-held assumption that adolescence sees the prefrontal cortex regions that handle executive functions 'waking up' and to the conventional wisdom that girls mature faster in this respect.
(Click on the thin rectangular box below this line if you want to start the movie)
A reward centre on overdrive coupled with planning regions not yet fully functional could make an adolescent an entirely different creature to an adult when it comes to seeking pleasure. In adolescents given a medium or large reward, the nucleus accumbens (part of the reward center of the brain) reacts more strongly than in children or adults
A speculation is that the lag between the frontal regions and the reward centre is an evolutionary feature, not a bug. "You need to engage in high-risk behaviour to leave your village and find a mate," and risk-taking soars at just the same time as hormones drive adolescents to seek out sexual partners.... in rodents, primates and even some birds, adolescence is a time of risky business, seeking out same-age peers and fighting with parents, which "all help get the adolescent away from the home territory".
"I don't think we can fight the biology of wanting to take risks and try on different identities. ...As a society, we can give kids creative, positive outlets that do not lead to irreversible mistakes...Attempts to push kids towards safe sex or pharmaceutical temperance shouldn't be expected to succeed if they simply explain consequences....Adolescents have some fundamental qualities to them that are not voluntary and not easily modified by rational, information-based interventions."