Nature by Ralph Waldo Emerson can be read by a range of age groups, but it does depend on their educational background, motivation, and the pedagogical techniques used. Emerson uses a wide range of vocabulary and relatively complex syntax, meaning that this is far more demanding reading than, say the anodyne "young adult" novels often assigned in dumbed-down classrooms.
In private or charter schools with strong academic standards, this would be interesting middle school reading, especially paired with Thoreau's Walden. At this level, you would need to provide a fair amount of support in helping students with vocabulary and training them to parse complex syntax and understand unfamiliar references; however, Emerson is no more difficult than Shakespeare, whose works can be read comfortably by motivated students at this level.
In public schools with large classes or many students reading below grade level, this would fit in an AP or honors class but probably not suit a regular curriculum. Any university student should be able to read it.