Looking at the various pages for the first, second, third and fourth ages, we see they are of varying length. I am wondering what is the definition of 'an age' in Middle Earth and how it is decided whether it is the end of an age?
The First, Second, and Third Ages were each roughly 3,000 years long, and each ended with a major cataclysm in which the primary source of evil was defeated. However, it is the cataclysm, rather than the duration, that determines when one Age ends and the next Age begins. If, for example, Morgoth had not been overthrown, the First Age would probably have continued indefinitely until he was overthrown, however long it took.
Unless otherwise noted, the quotations below come from The Tale of Years (Chronology of the Westlands) in Appendix B of The Lord of the Rings.
The first age ended when Morgoth was overthrown.
The First Age ended with the great battle, in which the Host of Valinor broke Thangorodrim and overthrew Morgoth.
The Second Age ended with the first overthrow of Sauron, servant of Morgoth, and the taking of the Ring.
The Third Age came to its end in the War of the Ring; but the Fourth Age was not held to have begun until Master Elrond departed, and the time was come for the dominion of Men and the decline of all other 'speaking-peoples' in Middle-earth.
We don't know much of anything about the following ages, especially how they ended. After the first few centuries, the records, which were already sketchy after the War of the Ring, simply stop.
According to Tolkien, we're probably living in the late sixth or early seventh age now, and roughly 6,000 years have passed since the fall of Sauron. However, he also said that this means that the duration of an Age has shortened; if the initial pattern had persisted, we would now be in the late Fifth or early Sixth Age.
From Letter #221:
I imagine the gap [between the final destruction of the Ring and the present day] 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 Second Age and Third Age. But they have, I think, quickened; and I imagine we are actually at the end of the Sixth Age, or in the Seventh.
[Thanks to TML for reminding me of the exact quote]
Note: It is not clear if the following are considered to be "Ages", but one or two periods preceded the First Age, not including the creation of Ëa (i.e., the universe). Collectively, they are referred to as the Elder Days or the Valian Years. They include the time of the Two Lamps (the first source of light, which Morgoth destroyed), and the Years of the Trees (the second source of light, also destroyed by Morgoth and Ungoliant). The Elder Days, or Valian Years, came to an end when the Valar created the sun and moon, after which Men awoke in Arda. The Elves had awoken long before, and had relied upon the stars for light while they were in Middle-earth, until the sun and moon were created..
There's a pretty solid answer in Wikipedia, namely:
In some texts Tolkien referred to the 'First Age of Middle-earth' or the 'First Age of the World' rather than the 'First Age of the Children of Ilúvatar'. These variations had earlier starting points, extending the First Age back to the creation of Arda, but consistently ended with Morgoth's defeat in Beleriand.
Tolkien wrote that the later Ages lasted about 3,000 years, though this duration was not fixed and he felt that the Ages 'sped up' over time. Each ended following the completion of some major event in the history of the Children of Ilúvatar.