Hmm, well, maybe, but I was definitely reading them at seven, especially the Marguerite Henry books as those were often bigger type and had lots of illustrations (especially King of the Wind, which I read to pieces...) I would never underestimate interest as a way to bump up a reading level, it can often work out that kids read a year (or two) above where they are generally, because they are interested in the topic. Mustang would probably be better read aloud, but I am sure there are also edited/abridged versions if they wanted to read the two versions side by side.