I think the age shouldn't be raised because when your 21 you are in university and you won't have time to get your driver license but if you do you probably won't even have the money to by a car. But if you are allowed to drive when you are 18 you would have more time to practice driving and if your lucky your parents will buy you a car.
As I said,first contention is a peer pressure. Many teenagers buy a car at eighteen years old. I"m sure that this pressure made them getting a car too. I mean when somebody shows a driving license for everyone to see and those who do not have a permits for driving a car will want to get a permit. At eighteen you may be able to get your driving license, but it does not mean that you can not get your driving license at 21 years old. Thus, changing the driving age to 21 will not be a dramatic change. As I said peer pressure really effects to other students because a part of them do not pay attention to getting their driving license until they see that other students already have a permit for driving a car.
Of course, it is from real life and I am sure peer pressure has a big influence.