Adult here has different meanings. One is fully matured growth or you should of possess age of 18 as prescribed by law, so first you must be in this qualification. By the way, legally to be an adult different countries have different age type. Legal adulthood age is also between 15 -21.
It is possible to be emancipated at different ages in different state. You have to prove you are capable of being an adult, such as job, roof, food etc.