As long as he or she meets the qualifications set by the car insurance company, teen drivers can remain on their parent's insurance policy well beyond their teen years - up to the age of 25. In the case of new teen drivers, it can be much less expensive to add the teen to a parent's policy than try purchase a car insurance policy of their own, because young drivers are the highest risk bracket of all drivers and personal insurance policies can be prohibitively expensive.
In order to qualify, the insured teen must reside in the same location as the parents, or live away from home in a dormitory or other sponsored setting that allows access to continuing education. College students are eligible for coverage on their parents' policy until they graduate or turn 25, as long as they remain in college. By contrast, an 18 year old who has a job and pays for their own residence is considered to be emancipated, and are responsible for paying for their own bills, including their own auto insurance.
From the point of view of an insurance company, having the teen listed as one of the driver of a the family car is preferable to insuring that teen in a car that only the teen is driving. Because of other drivers, the risk of insuring the teen is reduced by virtue of there being less time available for the car to be driven by the young driver. As the young driver gets older and more experienced behind the wheel, they are both better able to afford their own insurance coverage and a lower risk for insurance companies to bear.