What would be the appropriate age audience for The Great Gatsby if you were to look at it from its diction, tone, and syntax?
The use of sarcasm, double-meaning and allusion suggests a need for some verbal maturity on the part of the audience.
The proper audience for Fitzgerald's novel will have a sense of tragedy (what tragedy is formally and what it is on a more emotional level), an awareness that people are not always what they seem at first, and an understanding of the ways that we can both like and dislike people at the same time. Some of the power of the novel comes from its ambiguity.
This ambiguity plays out on moral and symbolic levels that may be beyond the grasp of younger readers. One must be capable of recognizing Nick's moral code as an open and dynamic code (he is coming of age in the novel) and see also that the absolutes of moral values that dominate the American landscape (think the eyes of Dr. T.J. Eckleberg) are influential but insignificant when considering the dramas of the human heart.
This complicated moral framework makes the book appropriate for adults, college age readers and relatively sophisticated high school students.