George Orwell's Animal Farm is typically studied on the honors 10th or regular 12th level of high school. It is appropriate for any student 8th or up who has a great grasp of the English language and a decent vocabulary.
Owell's topics include big government, conspiracy, communism, control through manipulation, etc. These are topics that interest and encourage discussion from higher level and mature thinkers--another reason for gifted or honors students from middle to 10th grade and regular 12th grade students.
Since Orwell is a British author, the book is typically not read on the 11th grade level in school since the junior year is usually reserved for American literature. This is not to say that juniors should not read the book on their own for pleasure and advancement.
As far as the readability rate, Animal Farm is a book that requires deep processing and questioning. It is not a book that is devoured in just a few hours. In order to get the most out of the message, the reader must question himself-- the characters, and the author's ideas--to see where it all fits into the big picuture on his own terms.
Word Count: 88,942 Reading Level: 8.9 Interest Level: 9+ Accelerated Reader: reading level: 8.9 / points: 17.0 / quiz: 5976 / grade: UG Reading Counts!: reading level: 8.2 / points:25.0 Lexile: 1090
Anywhere between eighth and tenth grade is optimal for the reading of "Animal Farm." The political symbolism and metaphor is a bit much for grades beneath eight, and by the time students reach 11th grade, they should have been introduced to many of the concepts that this novel involves.
The language in the book is well-suited for any of the grades mentioned above, although vocab. lessons will need to be incorporated for ELL, ESOL, or other students with learning differences. If I had to pick a grade to teach this novel to, it would probably be ninth, as a happy medium.