Though in centuries past 15-19 year olds were treated as adults, today we often paternalistically restrict their behavior because of "immature" brains.   An OpEd in Monday’s New York Times says 35-54 year olds actually behave worse:

A spate of news reports have breathlessly announced that science can explain why adults have such trouble dealing with teenagers: adolescents possess "immature," "undeveloped" brains that drive them to risky, obnoxious, parent-vexing behaviors. … But the handful of experts and officials making these claims are themselves guilty of reckless overstatement. More responsible brain researchers … caution that scientists are just beginning to identify how systems in the brain work. …

Our most reliable measures show Americans ages 35 to 54 are suffering ballooning crises: … 46,925 fatal accidents and suicides in 2004, leaving today’s middle-agers 30 percent more at risk for such deaths than people aged 15 to 19 … 21 million binge drinkers (those downing five or more drinks on one occasion in the previous month), double the number among teenagers and college students combined …

Overdose rates for heroin, cocaine, pharmaceuticals and drugs mixed with alcohol far higher than among teenagers. … More than half of all new H.I.V./AIDS diagnoses in 2005 were given to middle-aged Americans, … It’s true that 30 years ago, the riskiest age group for violent death was 15 to 24. But those same boomers continue to suffer high rates of addiction and other ills throughout middle age, while later generations of teenagers are better behaved. Today, the age group most at risk for violent death is 40 to 49, including illegal-drug death rates five times higher than for teenagers.

Strangely, the experts never mention even more damning new "discoveries" about the middle-aged brain, like the 2004 study of scans by Harvard researchers revealing declines in key memory and learning genes that become significant by age 40.