seven Ages of the world
In a letter to Rhona Beare on 14 October, 1958, Tolkien remarked that he imagined the gap between the War of the Ring and our own present
to be about 6000 years: that is we are now at the end of the Fifth Age, if the Ages were of about the same length as S.A. and T.A. But they have, I think, quickened; and I imagine we are actually at the end of the Sixth Age, or in the Seventh.
Some years later, Tolkien made an offhanded remark to an interviewer that 7000 or 8000 years ago would be an appropriate timeframe for imagining the events depicted in The Lord of the Rings (January 1971; BBC Radio 4 'Now Read On ...' with Dennis Gerrolt).
Six, seven or eight thousand years? The exact quantity is perhaps less important than the "ballpark" within which Tolkien seems to have been working. But more interesting still is Tolkien's periodization of time subsequent to the War of the Ring. We know when the Fourth Age began, but when did it end? More to the point: what event or chain of events marked its end? Similarly, what world-historical markers might Tolkien have attached to the Fifth, Sixth and Seventh Ages?
Martin Baker laid out his own periodization in the essay that christened this smial [Reunion 1], but the specific accents of that scheme are clearly governed to a large degree by Martin's personal vision and interests. This is perfectly legitimate, but what I would like to ponder here is the scheme Tolkien might have in mind, given the vision and interests that shaped him. Naturally, my reconstruction is just as subjective as anyone else's, but here it goes.
I imagine that, being a devout Christian, Tolkien would have envisioned some kind of bridge between his private mythology and the Judeo-Christian mythology to which he adhered. My guess is that the Fourth Age would have fed into the biblical myth of the antediluvian epoch in which Mankind grew more and more wicked, resulting in the biblical Deluge. (Consider, for instance, how already Tolkien conceived of things going bad in Gondor soon after Aragorn's reign in "The New Shadow"). One advantage to this periodization would be its convenience as a device for explaining the very different geography of the later ages.
On this hypothesis, the Fifth Age would have been the post-flood epoch, no doubt focusing in Tolkien's mind on God's covenant with Abraham.
The Sixth Age then falls right into place as that initiated by God's incarnation in Christ. (The centrality of this event as Tolkien's pivot of history is obvious, and is moreover alluded to in this capacity in the "Athrabeth" of Finrod and Andreth.)
What, then, of the "Seventh Age?" In my view, the most likely candidate is the "Machine Age," an expression which Tolkien used in his essay "On Fairy-stories." A survey of his published letters confirms the significance with which Tolkien viewed "the tragedy and despair of all machinery laid bare."
In a letter of 30 January, 1945, he writes to his son Christopher:
Well the War of the Machines seems to be drawing to its final inconclusive chapter ... leaving, alas, everyone the poorer, many bereaved or maimed, and millions dead, and only one thing triumphant: the Machines. As the servants of the Machines are becoming a privileged class, the Machines are going to be enormously more powerful. What's their next move?
If this reconstruction is accepted, it may serve as a basis for adjudicating among Tolkien's various alternatives in dating the War of the Ring. Because if the Deluge (with its traditional Judeo-Christian dating of 2348 BC) is taken to mark the end of the Fourth Age, and if the Fourth Age was, as Tolkien intimated in his letter to Rhona Beare, about the same length as the preceding two Ages (ca. 3000 years each), that would set the end of the Third Age at around 5000 BC, so that the War of the Ring would have taken place about 7000 years BP. This would also allow for Tolkien's idea that the Fifth and Sixth Ages were quicker (ca. 2000 years each).