If someone asked me to name the biggest difference between young adult and adult stories, I would say, “Death.” Yes, I know that there’s more to it, including the age of the character and the difficulty level of the writing; however, when it comes to the maturity level, death plays a big part in transitioning between children’s books, YA fiction, and adult novels (IMHO).
It really makes a whole lot of sense when you think about the intended audience. Besides the obvious physical differences, one major distinction between childhood and adulthood is the awareness of the harsher aspects of life – such as death. The older we get, the more we become aware of our own mortality and the mortality of everyone around us. Even when we don’t think about it, the knowledge is there. It’s like living in the city: we may get used to the noise, but it doesn’t go away. When a book has that feeling, that looming sense of inexorable death, it has a more mature, adult feel.
You see, kids still have blinders. They have an obliviousness to death. It’s not that death isn’t present – it simply hasn’t entered into their scope. They live in a bubble where ignorance truly is bliss. Until they experience it more directly (and that bubble is popped), they don’t develop that continual subconscious weight.
While books for young adults may brush against the darker aspects, they generally don’t linger. And the younger the intended audience, the more there’ll be a feeling of immortality. Most of the time, a young reader goes in knowing that the main characters are not going to die (even if that means they’re brought to life or saved by magic). It’s an implied promise in the story.
If you want a clear demonstration of the transition from YA to adult, look at the Harry Potter series (warning: vague spoilers!). It starts out at the younger side of YA with lighthearted overtones and the expectation that no one important will die (expectations that are fulfilled for the first 3 books). In the 4th book, the story begins to darken as the first character dies “on screen.” As the series goes on, the stories get more complex as the characters have to face the harsher aspects of their world. By the end, the books are more on the adult side of this conversation.
There are exceptions to both sides of this. Some genres of adult fiction may ditch the sense of impending death (some comedies or romance novels for instance), and some YA books have more mature themes and complexity. Others like the Harry Potter series may change as they go on. If you’re trying to make your work feel more or less mature, however, death is definitely something to think about.