When most American youths are fifteen and sixteen, they are still under the constant watch of their parents; every aspect of their life is analyzed. While this may be annoying to some teens, it does tend to keep them out of trouble. Most teens need the driving tips and lessons provided by their parents. A lot of teens are out on their own or in college when they turn eighteen. Therefore, their parents are not there to teach them how to parallel park or merge into a lane of traffic.
Being under your parents roof also provides extra motivation to stay out of trouble. Many parents tell their children that their license will be taken or they will be grounded if they get a parking ticket or get busted speeding. The never ending harass of parents tends to steer teenagers away from alcohol, drugs, and other disobedience issues. When you are on your own, your parents are not there to remind you to obey the speed limit and stay a few car lengths behind the car in front of you. These may sound like minor things, but if they are not practiced, you can get into a lot of trouble.
Another reason why it would not be wise to raise the driving age is that many eighteen year olds have schooling they need to get to and jobs they need to hold in order to support themselves or even their own family. It would be senseless to have a bunch of eighteen year olds out on the interstate going forty-five miles per hour or crashing into the median. Just the fact that they are eighteen does not automatically make them better drivers. They still need to practice as with any other task. It seems to me that the only effects of raising the driving age would be a lot of displeased young people and just as many car accidents as we have now.