This is an excellent question to consider with reference to this novel. Certainly, the world of the Greasers and Socs shows numerous characters who lose their innocence very quickly in life. If I were you, I would probably want to focus on Johnny, Darrel and Cherry.
Johnny of course is the character that interprets the Frost poem that assumes such importance in the novel. He is a character who, inspite of his youth and good manners, is haunted by being beaten up by a gang of Socs. His loss of innocence was actually initiated long before the novel began by the kind of treatment he receives from his parents. However, it is confirmed when he ends up killing Bob Sheldon and his resulting flight.
Darrel, as Ponyboy's elder brother, has had all of his hopes and dreams dashed by the death of their parents and the need for him to provide for his two younger brothers. We are told that Darrel could have been a Soc under different circumstances, and it is clear that he has the intelligence and ability to do so, but he is thrust into adulthood at an early age, and tries to perform the conflicting roles of father and older brother to Ponyboy, which makes him doubt Darrel's love for him. His innocence is lost by the harsh realities of poverty and circumstance, that have robbed him of his dream and consigned him to a life of menial work when he could have achieved so much more.
Cherry is an interesting character to consider for this essay question. Despite Ponyboy's ideas of life as a Soc, it is she that helps him to understand that life is hard both sides of the fence and that Socs have just as many (albeit different) pressures that threaten to crush them. Her loss of innocence is likewise sealed when her boyfriend Bob Sheldon is killed by Johnny. Although she recognises that he was a brute, at the same time she loved him and recognised how loving he could be. As she struggles to make sense of a world where such a thing could happen, she turns informant to help the Greasers and try to avoid further bloodshed.