Ages is the term used to divide and describe certain periods of history in Middle-earth. Ages do not have exact lengths of time. A new one usually starts after the current Dark Lord is overthrown and/or some catastrophic change happens to the world.
There are four Ages of Middle-earth:
- The First Age, which is covered by most of The Silmarillion. It begins with either the awakening of the Elves at Cuiviénen or the first rising of the Sun, depending on who you ask. It ends with the defeat of Morgoth by the Valar, the sinking of Beleriand, and the raising of Númenor.
- The Second Age, which is hardly covered at all, but what we have is in the last bit of The Silmarillion, Unfinished Tales, and the Tale of Years in The Lord of the Rings. The end of this Age sees the fall of Númenor, the Changing of the World, and the defeat of Sauron by the Last Alliance of Elves and Men.
- The Third Age, the end of which is covered in The Hobbit and The Lord of the Rings. It ends with the final defeat of Sauron and the flowering of the fourth White Tree in Gondor.
- The Fourth Age, which no one knows much about because it is only just beginning at the end of Lord of the Rings. It is presumably very peaceful, as there are no more Dark Lords to wreak havoc.