[Mithadan]

Welcome to the Barrow-Downs, Ar-Zigûr. You raise an interesting point.

When Tolkien began writing about what would become Middle-Earth, there were no "Ages." His mythology focused upon the Valar, the Elves, and the creation of the world through what eventually became the War of Wrath. He did not conceive of Numenor and the Second Age until later, and Hobbits, Gondor and Sauron (at least by that name) did not emerge until even later. So Tolkien did not have to account for the "Three Ages" until later in the development of his stories.

Early on, he conceived of the need to account for "Valinorean" time, as contrasted to Middle Earth time. He distinguished between the annals of Valinor and the annals of Beleriand. He realized that a vast amount of mythological time was needed to account for the creation of Arda, the struggles with Melkor, the arising and travels of the Elves and their dwelling in the West. Valinorean time grew with his mythos. But the early annals of Beleriand encompassed the time between the death of the Trees and rebellion of the Noldor, and the rising of the sun. The first span of time was not specifically described (and grew, in Tolkien's view, from a matter of years into tens or even hundreds of years). But after the rising of the sun, time in Middle Earth was calculated by "Sun Years." Some seem to equate the "Sun Years" portion of the annals of Beleriand (initially a bit over 250 years and later over 500 years) to the "First Age." But I am not sure that Tolkien himself ever did (and I'm not inclined to try to research this on a holiday weekend). There is no Tale of Years for the First Age. Nor, if I recall, did Tolkien ever clearly define what was encompassed by the "First Age."