Adult behavior includes adult thinking patterns, with enough experience to be sure that those thought patterns are not considered childish by or are repugnant to at least a sizable group of people who have reached the age that societies believe people to be responsible for their own decisions. It also includes adult actions that again, are not considered childish by or are repugnant to at least a sizable group of people who have reached the age that societies believe people to be responsible for their own decisions. It includes by definition, being responsible for a persons own decisions including legal decisions and I would argue, to be responsible for one's financial decisions. I believe that if someone is 25 years old and has someone paying their way through life is not fully adult, even if they could do so if they wanted to. Yes, full time students are not fully adult at that age if they are dependent on someone else. By this definition, someone who is running a household including bringing in the money, at 14 and makes adult type of decisions, is in fact, an adult. They may not be fully mature, but they are an adult. Since I live in a community property state and I believe marriage and civil unions to be partnerships, if a partner is bringing in the money, it is the same as the person bringing in the money.
Maturity has more to due with life experience. Someone who is 40 without much life experience is still somewhat immature, However, by studying, absorbing and understanding, they could still be mature.
Adult responsibilities: take care of self, take care of loved ones, promote good societal values, defend self, loved ones and society (note order). Privileges: right to have thoughts considered by other adults of society, fully able to take advantage of all rights granted in a society.