If the story had been told from an older person's perspective, part of the analogy that is the title would be lost. To kill a mockingbird is an analogy for to kill innocence, and even though that is a direct reference to Tom, it also references the kids. The kids were innocent, but they were exposed to the case and other crimes so their innocence is damaged. Also, the book wouldn't be the same if the narrator already had an understanding of how cruel the world could be. One of the biggest points of the book was Jems reaction to Tom being so harshly punished, but an adult would have expected it. Also, a big part of the book was about growing up and a mature narrator could not have made that point because they had already grown up.