Why can't you move out until you're 18?

Because in the eyes of the law, 18 is the age of adulthood and full responsibility. You can't enter into any kind of contract until the age of 18, so you can't get a lease, sign up for utilities, buy a car or get any kind of insurance. It's assumed that once you're on your own, you're supporting yourself and not living off of someone else's good graces, which can eventually lead to financial dependency on the government. Therefore, it's desirable for a juvenile to stay in school and graduate at 18 so that he/she can eventually go on to be a fully independent and financially responsible adult. Too many young people think moving out means freedom. They could not be further from the truth. There might be a temporary period where they can enjoy breaking all the rules they felt were uneccessary, but with every year of their life, they have more responsibility and less youthful freedom. They should enjoy the one time in their lives when someone else will foot all their bills and be their legal guardian.