What is the proper age from teenager to adults?

 In the Korea, when teenager turn 20, they usually graduate high school, become adults. Certificate of residence prove that teenager become adults. Boys and girls become adults at 20, but teenagers become adults at 19 from 2013, becuase teenager reach full growth early than old days. If teenager recieve certificate of residence, they don't pass birthday, then they aren't become adults.
 Finally, teenager become adults, they can a conrtact and a juristic act without parent approval. Differently the USA, in Korea, although teenager turn adults and become university students, most of they are supported by parents. Parents pay school expenses and live us together. Sometimes, some students earn their school expenses oneself and live in a dormitory or board oneself. Some people often live with their parents at one's thirties.
 We had a traditional coming-of-age ceremony long ago. These days, we have simple ceremony for teenager who become a adult. The date for ceremony is May third Monday, every year. Teenager recieve roses and a perfume from parents and a sense of responsibility.
 I think that teenager becoming adults at age 15 is so early. They have not enough experiences and education. To be sure, teenager are not become audlts perfectly at any age.  People in twenties are adults legally in Korea,  but becoming adults are subject to variation according to the circumstances. So, I can't say exactly when teenager become adults.