When a person turns eighteen does the fabled light of enlightenment come down and totally change the way you think, somehow transforming you into an adult? I don't think so. In this country the age at which you become a legal "adult" is eighteen, this means that you become a part of the true population of the United States, sharing the responsibilities of the masses, such as voting on important issues that you had no say in when you were seventeen and three-quarters.
Another version of the story is that you become an adult when you move out and begin to see the story of life for what it really is, my advise to you is to stay at home until you get tired of your parents giving subtle hints about you leaving their future guest room, don't become an adult until you have to, it's not really what you always thought it would be. .
When you become an adult you will acquire this thing called responsibility, which literally means that you are responsible for yourself. It is your responsibility to get up and go to work, your boss isn't going come to your house every morning and powder your face for you, or hold your hand while you make your decision on what shoes to wear. It is your responsibility to go to school there aren't anymore busses that will stop in front of your house every morning, wait on you until your mother hurries you out of bed, pours food into you, and boots you out the door. You will have to fight to balance school and work, because you can't just go home every day after school and expect the fridge to be full, food costs money, money requires a source of income such as a steady job. Adult life is not simple, it's not fun, it is life. .
There is no way to tell when a person becomes a true adult, to me you could be well past your teens and still be a child in the eyes of some people. To become an adult there are no big exams to take, nor is there some physical standard that makes you an infallible adult, eligible to do everything that you have always wanted to do.