It being strictly a history of a boy, it must stop here; the story could not go much further without becoming the history of a man. When one writes a novel about grown people, he knows exactly where to stop -- that is, with a marriage; but when he writes of juveniles, he must stop where he best can."
As it is solely a tale of a boy, it must conclude here; the story cannot extend much further without transitioning into a man's history. When one writes a novel about adults, they know precisely where to end -- that is, with a marriage; however, when writing about children, they must stop at their best point.