Throughout the course of puberty, teens' moral development advances just as surely as their bodies are changing. From the start of early adolescence (around 11-13 years old), teens begin to think abstractly. This new ability leads them to begin to question rules and standards, whether from school, religious authorities or parents, which they had previously easily accepted. By middle adolescence (around 14-18 years old), teens have begun to develop their own moral code. Their behavior early on during middle adolescence may not be consistent with some of their moral conclusions. This isn't hypocrisy; they really just haven't made the connection yet. It's the reason they may genuinely believe smoking is bad, but they'll try it anyway. They will usually be more likely to match their behavior to their beliefs by the end of this period. As they move to late adolescence (around 18-21 years old), they will begin to develop their moral ideas about the world, thinking about such concepts as justice, peace and patriotism.
This progression was formally codified by Lawrence Kohlberg, a 20th-century developmental theorist who identified six stages of moral development. According to Kohlberg, early adolescents have usually reached the third stage of moral development: the Conventional Level of interpersonal cooperation. The Conventional Level means they understand morality as determined by social convention and wish to conform to be a "good person." Stage four is the social-order-maintaining phase of the Conventional Level. This means that teens begin to assess morality by what is best for the most people. As teens get older, they enter the Principled Level of morality, which includes the fifth stage of moral development -- the social-contract orientation. During this stage, teens start looking outside their needs to assess morality. They also start to think more abstractly about morals: not just accepting rules because they are social norms, but assessing a moral principle through the lens of their values. Lastly, the sixth stage of moral development, also part of the Principled Level, is when teens begin contemplating high-level abstract concepts like respect, equality and justice. This sixth phase is called the universal ethical principle orientation.