Or, well, not fully developed. Teenagers, that is. The Washington Post today reports that a 2004 NIH study "suggests" that "the region of the brain that inhibits risky behavior is not fully formed until age 25." This, we are told, has "implications for a host of policies, including the nation's driving laws." Here's the link, but you have to register.
Oddly, the article doesn't state what the parameters of the study were (apparently involving MRI scans), although they did state generically that "brain-imaging research," indicates "no proven correlation between brain changes and behavior."
Also oddly, the article quotes the leader of the study as saying "[w]e'd thought that the highest levels of physical and brain maturity were reached by age 18."
Obviously, the good doctor has never been to a frat party...(Yes, I know, I'm guilty of conflating brain maturity and behavioral maturity, too, but the line was too good to pass up.)
Finally, the article also notes that scientists have recently discovered that teenagers are willing to succumb to peer pressure ("take more chances when friends are present") and that "the judgment of teens deteriorates with distractions."