Permitting young people to drive at the age of 15 or 16 allows for novice drivers to gain experience while still under the supervision of their parents. In addition, driving at 15 or 16 provides a young person the ability to enjoy increased independence and to take on more responsilities as an individual moving towards adulthood.
No, the age of 18 is not a better minimum legal driving age than 15 or 16. Teenagers need to earn money for college in the last couple years of high school, and the ability to drive is necessary to hold a job. Teens who are unable to drive must rely on their parents for transportation, which creates an unnecessary burden for the parents.
The age of 18 is a pivotal year for many young people. Many of them graduate from high school and begin college when they are eighteen. Some register for the military. They move away from home and their parents for the first time. Adding another rite as extreme as driving to that already taxing year may prove to be too much and it may actually interfere with the proper development of driving skills. If the driving age was pushed to 17, for instance, that still gives teenagers a bit more time to mature and allows them to obtain their license prior to the all important 18 mark.
Children able to drive at 15 or 16 are able to assert more independence over their lives. They require less of their parents which helps them to learn personal responsibility. It also helps to free up parents' time, because their children can drive themselves to and from school and to their friends' houses.