Well, perhaps Modernism is really best understood as a logical development of the Renaissance, whose “moment” is the true beginning (as if a single moment can define it) of this third age. But if you draw a rough analogy to the development of an individual human, maybe it would be:
1: Human life unaware of consciousness. Infancy-childhood: human history up to the Renaissance. (Sorry, Age of Pericles; I know you really belong later; you jumped the gun.) Prehistory.
2: Human life aware of consciousness. Adolescence: human history Age of Pericles-Modernism. History.
3: Human life aware of the consequences of the awareness of consciousness. Adulthood: Modernism on. Will there be an early senescence? Probably. Metahistory, or Historicism.
This looks like college-student late-night talk, I know. And it’s influenced by a book many think of as dubious, Julian Jaynes’s The Origin of Consciousness in the Breakdown of the Bicameral Mind (Houghton Mifflin Company, 1977). Jaynes, an American psychologist who published no other book, held consciousness to be a cultural construct, not an autonomous function of the individual human “mind.” (Those are precautionary quotes; let’s not take up the question of “mind” here.) As I recall — I read the book a long time ago, and haven’t revisited it — he takes care not to fix an exact date or cultural “moment” at which this construct appears; but he does identify it, in the Mediterranean context, with the Homeric age, arising between the composition of the Iliad and the Odyssey.
Jaynes cites evidence for his hypothesis in linguistic and economic models, among others, and finds in psychological evidence of our own time parallels to the historical (and prehistorical) patterns of unconsciousness, consciousness, and their interfaces. I found his discussions persuasive; and am particularly interested now to read scientists arguing for the abrupt big changes that can determine human behavior, collectively (politically) as well as individually. Nassim Taleb, in The Black Swan, discusses such cataclysms in the economic sphere; the research geologist Dave Wahl, in the current issue of Terrain, discusses them with respect to climatological changes. (Taleb; my earlier blog on Taleb ; Wahl.)
Historicism is inevitably recursive. I’ve always loved Francis Ponge’s description of recursive irony — he cites Maurice Ravel's La Valse — as typical of periods "when rhetoric, dying, examines itself.” (Lane Dunlop's translation, in Soap [London: Jonathan Cape, 1969]; in the original Ce genre est particulier aux epoques ou la rhetorique est perdue, se cherche: [Le Savon: Paris: Gallimard, 1967]). [Cited in my article “What's the Matter with Today's Experimental Music? Organized Sound Too Rarely Heard,” Notes, December, 1993.]
To continue woolgathering: there may be a parallel between all this and the inevitable process which finds "art" declining from Religion to Art to Entertainment. (See Walter Benjamin: The Work of Art in the Age of Mechanical Reproduction ; and see also Wikipedia on Walter Benjamin. Come to think of it, Benjamin himself should be added to Jaynes, Cage, Duchamp, and many others as a seminal organizer of aware-or-consciousness consciousness.)