According to Jean Piaget's theory of cognitive development, you're an adult when you reach the "formal operations period", at around 12-15 years. This means a person develops the ability to speculate and think in abstract terms.
Lawrence Kohlberg's theory of moral development states that the final stages of moral development are the "Postconventional Level" that takes place (or can take place; many people never reach the postconventional level) from 13 years. This means a person doesn't act out of fear or a need for approval, but actions will be guided by a sense of right and wrong and the development of a moral conscience.
Abraham Maslow's theory of social development states that the "mature adulthood" stage occurs when there is a need to answer the question, "who am I?" and develops potential. Also, a need to place long-term benefits to self and society before short-term pleasures. and a need for a secure sense of identity that can lead to 'self-actualisation', or 'a true sense of understanding who one is and what one believes'.
I know it's an adult only poll, and that's why I don't intend on voting, or giving any real opinion. I'm not trying to prove anything, but I thought that you all may find it interesting to read the opinions of sociologists, though