The old joke about teenagers moving out while they still know everything still applies, but now, there's scientific evidence to back up what every parent already knows: teenagers have no forethought and don't consider the consequences of their actions.
More interestingly, to me, this research suggests that the decision-making center of your brain--the prefrontal cortex--doesn't fully develop until the late 20s. By that time, people have been through college (or dropped out), gotten married and had babies (not necessarily in that order). Is it any wonder young adults get in so much hot water so quickly with finances, careers, dating, and family? According to the science, they simply aren't mature enough to handle making these decisions. Yet the same guy who spends all his money on gaming systems has the same voting rights mature adults do.
Ann Coulter has proposed repealing the 26th Amendment, based on the crappy voting record of young adults. But given the research that shows 18-year-olds don't have the mental capacity to make good choices, maybe it isn't such a bad idea.