So, my niece just turned eight and I got her the chapter book version of 'The Lion King' for her birthday. She's been reading chapter books for about a year now, stuff like 'Magic Tree House', and she tore through this one in a couple of afternoons. The font is bigger than a typical novel, and there are still some illustrations every few pages, which really helped her confidence.
I'd say it's a solid bridge for kids who are moving beyond picture books but might not be ready for denser middle-grade novels. The vocabulary is manageable, and the story is familiar, which reduces frustration. If a child is a strong reader at six or seven, they could probably handle it, but for most, I'd peg the sweet spot at seven to nine years old. My nephew, who's ten and reads a ton of fantasy, thought it was a bit too simple for him.