According to the D'ni cosmology, the Creator Yahvo created the Terokh Jeruth, the Great Tree of Possibilities, which has an infinite number of branches and leaves, each leaf being one Age of the multiverse. Each Age is distinct and unique although an Age might resemble another like a parallel universe. However not all worlds of the multiverse are viable nor all can support life.
The forefathers of the D'ni, the Ronay of Garternay, developed what they called The Art (Gestoy), which allowed them to describe a world in Descriptive Books, that could create a Link between Ages, allowing people to travel to the Ages described in each Book. The Art is followed a complex set of basic principles, like avoidance of contradictions, that ensured stability and viability; failure to follow the proper rules would describe, and link to, an unstable or near-death world.
Many Ages, especially those written by Atrus, are written as or with central themes (Haven and Edanna are jungle-based Ages). According to Atrus, these themes give the ages balance, though in creating Releeshahn his intention to create a balance of all plausible themes that facilitate the growth of civilization.
The D'ni word "Age" doesn't refer to periods of time, but is a synonym for what we would call a "Universe". The ter might refer to the infinite expanse of Time of a Universe, from uttermost past to ultimate future, defining and encompassing therefore all of its existence. Properly the term "Age" thus refers to the whole universe in which the usable place (e.g. an island) belongs. In this sense, the Age of Myst refers not to the island of Myst, but the whole planet and universe wherein that island exists.
In practice, the term "Age" is not used in the cosmological sense, but rather loosely to refer specifically to the visitable place of the Age, whether it is an island, a limited space, or the civilization the visitor has access to. The text of the Myst III: Exile - Prima Official Guide equates the collapse of the Lattice Tree with the destruction of the Age of Narayan; the collapse of the Tree would destroy the Narayani civilization but it is unlikely it would signify the destruction of the planet it was standing on, or the universe it belonged to; therefore the word "Age" here is used more freely, as the scope of Atrus's writing or consideration.
Almost all of the Ages featured in the games look like islands, simply because islands (or any limited place) are much easier to design and render for the games. This could be true for the D'ni as well, as describing an isolated, autonomous place like an island (or the Cavern) would be easier than bothering with what would exist beyond. Vast Ages such as Terahnee, Er'cana or Releeshahn, were probably exceptional masterpieces.
Most individual Ages were christened according to the characteristics of the islands visited.
- Cyan's information on the D'ni (old)
- D'ni Desk Reference- among others, hosts a very comprehensive list of Ages