Good question! Depends on the kid, but probably 4 to 8 or so? It's a picture book with a sentence or two on each page -- not a bunch of huge chunks of text. And there are some slightly scary brief moments with the witch, bee, and dragon, but we tried not to make the character designs to scary (the giant bee is actually adorable). For reference, the moment with dragon flying over the fire in the image above is probably the most intense moment in the whole book.
If you're reading aloud to a child, I imagine even a 2 or 3 year old would enjoy it.