Based on Harper Lee's To Kill a Mockingbird, what will Scout and Jem gain and what will they lose as they get older?

In the final chapter of Harper Lee's To Kill a Mockingbird, just after Scout escorts Arthur Radley home and sees the world from his perspective for the first time, Scout reflects to herself, "Jem and I would get grown but there wasn't much else left for us to learn, except possibly algebra" (Ch. 31).

Scout's thought reflects the fact that she and Jem have learned so much over the past three years. They've learned what it is to be brave, and Atticus's most valuable lesson, that "you never really know a man until you stand in his shoes and walk around in them" (Ch. 31). Because of these lessons, Scout and Jem will retain their abilities to let go of prejudices, to see things through others' perspectives, and to bravely behave with compassion contrary to the way the rest of society behaves. However, during the trial, they also learn what it is to become desensitized to the world, and it is, sadly, their sensitivities that they will inevitably lose.

During the trial, when Scout must take Dill outside because he starts crying during Tom Robinson's cross-examination by Mr. Gilmer, Mr. Dolphus Raymond makes some revelatory comments about Dill's sensitivity. First, Mr. Raymond acknowledges that Dill is crying because he perceived that Mr. Gilmer's antagonistic treatment of Robinson was unjust, and Mr. Raymond agrees with Dill, saying, "I know what you mean, boy ... You aren't thin-hided, it just makes you sick, doesn't it?" (Ch. 19). However, Mr. Raymond further philosophizes that only the young and innocent children are able to understand and respond to the injustices of society. He further makes the following revelatory statement about Dill:

Let him get a little older and he won't get sick and cry. Maybe things'll strike him as being--not quite right, say, but he won't cry, not when he gets a few years on him. (Ch. 20)

In other words, the more we witness the injustices of society, the more we grow desensitized to those injustices. And, though we still recognize they are wrong, they do not break our hearts in the same way they break the hearts of innocent children. Atticus later echoes Mr. Raymond's sentiment when, after Jem cries as a result of the guilty verdict, Atticus comments that, each time a jury behaves unjustly, "seems that only children weep" (Ch. 22). Dill even fulfills Mr. Raymond's prophecy of growing desensitized when he does not cry after witnessing Helen Robinson's response to the news of her husband's equally unjust death.

Hence, while Jem and Scout will retain their abilities to recognize and fight against the injustices of the world, they will sadly lose their sensitivities to such injustices and will no longer cry like innocent children over injustices.