Once you finish high school, you are no longer a minor, and you should learn that that means proper responsibility. Teens nowadays rely off of their parents to much, no matter how rebellious they become. Apartments aren't to much today, and the work force always needs more hands. Teens are helping themselves and the economy by moving out. It can also help their parents, who can finally get a financial break, which they haven't been able to get for the last 18 years.
They usually expect free meals and no rent. Sometimes, Mom and Dad still clean up their adult children's mess. It's easy to see how these expectation arise; after all, the parents have always taken care of their children. But it is unfair to expect parents to continue to support their adult children. But it is unfair to expect parents to continue to support their adult children in these ways. Parents have done their job by successfully guiding their child to adulthood, and probably to college. They should have to continue that job indefinitely.
It would be idealistic for a young adult to find a place of their own once they finish high school; however, it is not always possible. I am talking about financially (though there are special cases that are physically and mentally as well). Sometimes the best option is to stay with parents for a few more years, but this is not to say to become a lazy leech. Get a job (if they don't already have one) and go to school.