What are the criteria that the Wise use for deciding that a new Age has begun?

Question

Details taken from the Tolkien Gateway

The First Age ended with Morgoth's overthrow - a truly epic event which seems appropriate as the end of an era.

The Second Age did not end with the sinking of Númenor, and the reshaping of the Earth from a flat one where Valinor could be reached by sailing the sea to a round one with Valinor accessible only through sailing the Straight Road (world-changing events, by any measure!). Instead it ended over a hundred years later, with the first overthrow of Sauron.

The Third Age didn't end with the final overthrow of Sauron, though - it ended when the Ringbearers left Middle Earth.

What reasoning led to this divvying up of time, which sometimes follows hugely dramatic events, but skips others (the Reshaping of the Earth), and sometimes picks much less dramatic (though still significant, I'll admit) events? Do the Wise think that the Reshaping of the Earth is less important than the (temporary) defeat of Sauron?

Answer 1

Basically, a new Age begins after the old Age is fully ended.

Tolkien did not precisely define the end of the First Age, but some of the last events explicitly described as being in the FA were the trial of the defeated Morgoth in Valinor and his ejection from the universe, and Maedhros's seizing one of the Silmarils and subsequent suicide. It also appears that the great sailing of Elves into the West was very late FA. OTOH, Tolkien says that the sailing of Men to Numenor was at the start of the Second Age.

So the FA ended only after Morgoth was defeated, the Silmarils found their long-time home, and most of the Eldar departed Middle-Earth. At that point, all the FA's main protagonists' stories were complete.

The SA was the age of Numenor, starting with the sailing of Men to Numenor, which is the first event clearly described as part of it. It ended not with Numenor's downfall, because that left some pretty serious unfinished business: Sauron and the remnants of Numenor had to finish the fight, which they did with Sauron's final defeat and loss of the Ring (while still on his finger). (JRRT explicitly says this ended the SA and began the Third.) At that point, all the main protagonists' actions relating to the climactic battles were over.

The big theme of the TA was the battle between Men and Sauron (or his creatures) and, like the FA, the TA did not end with the defeat of the Big Baddie, but only when the protagonists -- the Bearers of the Three and the One and the Wizards -- had sorted themselves out and Middle-Earth became the Dominion of Men (and Dwarves (and Hobbits)).

You can see the three Ages of M-E as being the three great stories: Elves vs. Morgoth; Numenor, Sauron and its Sauron-caused downfall; Men (and Dwarves (and Hobbits)) vs. Sauron. And the Ages end when those stories end.