The differences between middle grade, YA, and adult novels?

What is it that really separates the age-genres?

The age of the protagonist is one factor, but Stephen King has child protagonists all the time and those books don’t “feel” like middle grade.

The content is obviously another factor, but I realized while reading a MG book that this doesn’t fully cover it either. Having a child character encounter a dangerous man in a dark alley with a bat with nails in it could feel very different depending on whether the book is MG, YA, or adult.

Another factor might be tone armor. If you set up early on that your characters are not safe, the story will lean towards adult.

Another factor might be popular trends. If your story has a teenage female MC in first person present tense, it will feel like YA from that alone.

Are there any others? Am I missing anything?