There is no single point at which we can say that all people reach adulthood. We sometimes define adulthood in terms of reaching a particular age, but not all people are adults at exactly that age. We sometimes say that we become adults when we achieve some change in our lives like when we graduate from college, get our first job, or get married. However, these definitions do not apply to all people. Adulthood is something of a social construct and different people can reach adulthood at different times.
One factor that can affect our definition of adulthood is our social class. This is not the only factor that affects how we define adulthood, but it is the one that you ask about in this question. In general, in the United States, people of the middle and upper classes believe that they become adults later than people of the lower class. This is generally because of the difference in how much education people of various classes expect that they will get.
It is very hard to be a full-time student and be considered an adult. High school and college students generally cannot support themselves financially because they are too busy with school to have full-time jobs. High school students in particular have to follow all sorts of rules laid down by their schools. The same is true of college students who live in dormitories. When you do not support yourself economically and you have to follow rules (other than the laws we all have to follow) that tell you how to live your everyday life, it is hard to say that you are an adult.
Because of this, people from lower classes tend to define themselves as adults sooner than richer people do. People from the lower classes tend to end their education earlier and get jobs earlier. Therefore, they are more likely to define adulthood as something that comes when they graduate from high school or perhaps even when they drop out and start working full-time. By contrast, people from the richer classes are more likely to say that they reach adulthood when they graduate from college or when they get married.