The first of the four Ages chronicled by Tolkien. Unlike the Second and Third Ages, there is no detailed chronicle of the events of the First Age, at least in any canonical sources, and so some dates must be inferred from references in the text - hence many dates in this Age are necessarily approximate. We do have two sources for dating this period, The Annals of Aman and The Grey Annals, in volumes 10 and 11 of The History of Middle-earth respectively. While not formally canonical, in the absence of any other comparable material, these sources are especially useful in dating events of the First Age. This entry deals with events of the First Age of the Years of the Sun; for earlier events see the entry for Years of the Trees, and for the rationale behind this distinction, see the essay Dating the First Age below.
Establishing the point where the First Age began is not as simple as might be imagined. The only really definite statements are in the drafts of the Appendices to The Lord of the Rings, which originally included this comment, relating to the time of the awakening of the Elves:
The History of Middle-earth volume 11, 3 V The Tale of Years
According to this dating, the First Age began about 4,312 years before the rising of the Sun, making the entire Age some 4,902 years long. This calculation is backed up by a further comment from other drafts of the Appendices:
'The First Age was the longest.'
The History of Middle-earth volume 12, 1 VI The Tale of Years of the Second Age
It's perhaps notable that no direct comment on the beginning of the First Age, or its length, survived into the published version of the Tale of Years (which indeed does not attempt to provide a chronicle of the First Age at all). It's hard to be sure why this should be, but part of the problem was doubtless the complication of having two dating systems: one before the rising of the Sun, and one after.
The whole notion of measuring time in 'years' is of course dependent on the existence of the Sun, so before it was created, a different dating system was needed. This worked on so-called 'Valian Years', dating from the making of the Two Trees; on this system the first Elves awoke in 1050. However, Valian Years were rather longer than solar years, so 1050 is equivalent to about 10,062 conventional years (actually there are various ways of making this calculation, so this is necessarily an approximation). We might imagine that the beginning of the First Age would restart the count of years, but in fact that did not happen until the Sun was created 1500 Valian Years (about 14,374 solar years) after the making of the Trees. From that point the First Age is dated in normal years.
So, a full chronicle of the First Age would involve two dating systems, using two different types of years; first a period of 450 Valian Years (4,312 solar years) starting in (Valian Year) 1050, then a period of 590 solar years starting at year 1 (the rising of the Sun). Given this level of complexity, a Tale of Years of the First Age would be unwieldy, to say the least.
A simpler solution is to follow the approach suggested in the Reader's Companion to The Lord of the Rings:
'...in Tolkien's mythology Men awoke only at the beginning of the First Age when the Sun first arose in the heavens.'
The Lord of the Rings: A Reader's Companion 7 In the House of Tom Bombadil
It's difficult to support this statement from Tolkien's known writings, though the authors of the Reader's Companion, Wayne G. Hammond and Christina Scull, were also editors of the 50th Anniversary Edition of The Lord of the Rings, so it's conceivable that their comment is based on material not in the public domain (if so, they don't mention the source). Nonetheless, taking the rising of the Sun as the beginning of the First Age simplifies matters considerably, essentially dividing the full Age into two sections (the long Years of the Trees and the much briefer First Age of the Sun). This makes dating the First Age a straightforward matter of using solar years from I 1 (which is the convention used throughout this site).