S. E. Hinton's The Outsiders is a coming of age story which follows Ponyboy and his two brothers. The Curtis' know about challenges and struggles--having to care for one another since the death of their parents. given the circumstances they are forced to face, the boys have been required to grow up far too fast.
In regards to Ponyboy saying that "he" has grown up too fast, one may interpret this as "he" referring to himself or "he" referring to another boy. Since Ponyboy never directly refers to the fact that he, himself, has grown up too quickly, one can only assume that the "he" in question is another character.
At one point in the text, Ponyboy is speaking directly to this idea. He believes that Darrel, or Darry (his oldest brother), has aged far beyond his years.
"But then, Darry's gone through a lot in his twenty years, grown up too fast."
Here, Ponyboy is defending his brother's cold demeanor (stating that he is only like he is because of growing up too quickly).