What is the most important life lesson that Ponyboy learns in The Outsiders?
I would say that the most important life lesson that Ponyboy learns has to be in the last couple of pages of the story, when he reads the final letter that Johnny wrote to him that includes Johnny's analysis of the poem by Robert Frost, "Nothing Gold Can Stay." In this letter, Johnny encourages Ponyboy to not be limited by the identity that others give him and he reminds him that there is still "lots of good in the world." This leads Ponyboy to contemplate the situation that is his reality, with hundreds of boys and girls divided and identified based on their wealth or lack of it and where they live in town. Note what he says about this situation:
Suddenly it wasn't only a personal thing to me. I could picture hundreds and hundreds of boys living on the wrong sides of cities, boys with black eyes who jumped at their own shadows. Hundreds of boys who maybe watched sunsets and looked at stars and ached for something better. I could see boys going down under street lights because they were mean and tough and hated the world, and it was too late to tell them that there was still good in it, and they wouldn't believe you if you did.
Ponyboy concludes that these boys need some help to tell their side of the story so that these boys would not be judged so quickly and to give them hope. Ponyboy realises that it is possible to defy the identifications that are given in this novel and which are shown to be so harmful: greaser or soc, and that these labels can be transcended.
Ponyboy is protected and sheltered by his brother and friends. The moment he chooses to save the children in the church is a pivotal moment for him. He finds himself, and his strength, but ends up losing two friends as a result. Johnny expresses pride in their decision, and reveals to Pony the meaning of the Robert Frost poem by inferring that the children's lives were worth more than his because they were young and innocent, and had more to offer. He dies with honor. Dally expresses anger and regret about their decision which eventually leads to his death as well.
Pony learns that we can make a big difference in the world and our own lives by the choices we make, and possibly control our own destiny. He also learns that he does not have to fit into the mold that his home and surroundings assign to him.