Adulthood has been way better than childhood for me. My teen years were some of the worst in my life. Now I have the power to decide things for myself, and the ability to live alone without people pestering m or moving my things around; my little apartment is a safe haven for me, where I can easily recharge before I go out to face the world again.
Now, granted, the transition is tough. From being a kid to being an adult can be really difficult, especially if--as I was--you're missing some of the skills you need to live as an adult. Missing skills can be learned, but it would be best if you learn them before you leave, because it's possible to crash pretty hard, depending on just what you're missing. If your parents are overprotective or don't realize that what's easy for other kids to learn, you might need help learning, then missing skills are likely and it's time to start yelling about getting somebody to teach you. The uber-skill, of course, that's most important, is knowing how to solve the problem of missing skills in the first place. Finding a person or a book or maybe an Internet article to either help you or teach you or make you a list of steps for something you can't do yet is something that's important to know. That, and self-advocacy (that is, knowing what to do about prejudice, knowing how to access services in your area, and knowing how to work the system in general), are probably the most important things you need to know before you leave.
That said--it's not uncommon, and not wrong, to stay past 18. If you do, have a plan in mind as to what your goals are now that you've graduated from school, whether that means college or vocational school or just learning more about how to do things before you leave. I highly recommend, if you can, getting a job (at least part time) and paying rent to your parents. This is a HUGE signal to them that you plan to move out and either support yourself or get your own arrangements, and do not intend to be a child forever.