While Stevenson is noted for writing children's adventures like Treasure Island and Kidnapped, the novel The Strange Case of Dr. Jekyll and Mr. Hyde is categorized as an adult horror novel. The striking thing about the origins of this story is that it was inspired by a nightmare from which Stevenson awoke screaming. The dream was of a villainous monstrous transformation induced by a white powder. One can only speculate that his fear resulting from the dream convinced him that adults were the proper target for such a frightening horror tale about the consequences of rejecting the need for controls on the powers of evil.
The most significant textual evidence that this is a story for adults is that much of the story revolves around a philosophical discussion of the nature of good and evil as in Jekyll's letter:
it chanced that the direction of my scientific studies, which led wholly toward the mystic and the transcendental, re-acted and shed a strong light on this consciousness of the perennial war among my members. ..., I thus drew steadily nearer to that truth, [...] It was on the moral side, ... that I learned to recognise the thorough and primitive duality of man; ... the two natures that contended in the field of my consciousness, ... I was radically both; ..., I had learned to dwell with pleasure, ... on the thought of the separation of these elements.
The internal debates within the text regarding the nature of good and evil are of an esoteric nature and not composed for the consideration of any but an adult mind. Another textual proof that it is a story for adults is that there are no youthful heroes or heroines: the characters of interest are exclusively adults, like the intended audience.