An interesting question. The most basic answer is, we don't know what happens beyond the end of the book. Almost as basic is the answer, they grow up.

However, to take the question further, we can see that Scout and Jem have both learned a lot by the end of the book. They are more mature, and they've been through the fire of injustice. Scout has had her life threatened, and was saved by Boo Radley. As a result, she's learned that the unknown can be kind, and the known (Mr. Ewell) hateful. Through it, she's become ever more part of her community, and come to love those close to her more profoundly.