They sense their own capacities and so are ready for life's challenging tasks. Social factors are also at work. The I am capable attitude is stimulated by their being welcomed into society at large. Permitted to go off to college or their own apartment, to marry, to vote, to buy alcoholic beverages, they are now finally on their own. Inner forces combine with new social roles.
Young adults seem to face the world in a way that is distinct from their adolescent days.
It is not that they were unexposed to world conditions during their teenage years. The difference is that their perception has changed. Unlike young adults, teenagers look at the surrounding world in terms of themselves and their impact on others. The technical term for that perspective is narcissism. Not to be confused with selfishness, it refers to the way in which youths see most everything in relation to the self.