According to SearchLit.org, George Orwell's 1984 is appropriate for most fourteen to sixteen year olds (ninth and tenth graders). This rating was the result of averaging eight different readability tests. The link below provides the test results for each of the different trials applied to determine readability.
The first paragraph of the novel scores an eleven on the Flesch-Kinkaid scale. This scale examines the sentence length and number of syllables per word. The higher the syllables and number of words in a sentence increases the grade-level readability.
While some texts are defined by standardized readability scores, teachers and districts may not agree with the scores texts are given. In some cases, 1984 has been taught as early as seventh or eighth grade. In other cases, some schools have chosen to teach it in senior AP (advanced placement) classes.