While the expression "Sixth World" is widely used and there are many references to at least one previous age of magic, there are actually very few sources that address the idea of cycle and their length. Ehran's speech titled Humans and the Cycle of Magic - two versions of which exist - is practically the unique source material.
http://www.amurgsval.org/shadowrun/imports/cycleOfMagic.htmlhttps://web.archive.org/web/20080317094824/http://www.shadowrun4.com/fiction/fiction1.shtmlFor the record,
Dawn of the Artifacts 3: Darkest Hour features Aztechnology-employed archeologists work putting Atlantis destruction in 9564 BC, contradicting the 3113 BC date Ehran uses. On the other hand,
Artifacts Unbound have several magical artifacts dated back to "only" 550 BC.
It's not even clear if the expression "Sixth World" came into use shortly after 2011 or later.
Shadowrun, first edition, says "some mystics" refers to what the Mayan called the Sixth World but does not state when they started doing so.
Sixth World Almanac mentions Ehran mentionning his 'controversial "Sixth World" theory' for the first time in a book published in 2043.
It is pretty well established that there was a period of no-magic, that came before what we call the Sixth World, so it makes sense to call it the Fifth World. And there are big flying lizard talking in your head that says they used to live in a previous period of magic and have been sleeping since, so it makes sense to call that previous period the Fourth Age.
But the reason why people call one the Sixth World in the first place is, as far as their statement goes, because the Mayan did it. Ehran (as far as we know, based on the HatCoM speech) doesn't make such statement, but does not give an alternative explanation as to why he call thel the Fourth and the Sixth Age. If the "Sixth World" usage predates Ehran's speech and book, he might actually just trying not to confuse his audience.
(See also a my
previous post regarding Ehran's speech accuracy)
As far as I remember, Earthdawn never refers to its period as the Fourth something. An older Age of Dragons is mentionned, but I'm not sure there are any mention of a non-magical period in-between. However, Dunkelzahn is said, as one of the oldest great dragons, to have hatched during the Age of Dragons, while he states in Shadowrun-era media he is 15,000 years old.
The most interesting part, in my opinion, is the lack of direct witness. While Ehran is born during the Earthdawn-era, he asserts the threshold between magic appearance and disappearance is about 5200 years. But personnally, he only has witnessed the beginning and end of the (so-called) Fifth World. And he wouldn't even know the later when he was, as he claims, designing the Egyptian calendar. It also assumes he was able to never lost track of time over five millenias (yes, he is immortal, but that doesn't mean he has eidetic memory) - his best bet would probably to carve a celestial map in a stone somewhere that he could refer to later to calculate how many times has passed (also works for dragons, which otherwise would have abolutely no way to know for how long they have slept). So if it has any truth, Ehran's statement of a 5200 years cycle must be based on someone else's information. You could not assert a regular cycle length before recording at least two entire ages - that is, by the end of the Fourth World for someone born during the second.
One of my favored theory follows the idea that the cycle of magic is altered by the use of magic. If there are many magic users, and/or if they use powerful magic (like the Ghost Dance), the magic level will move faster, possibly higher, and may later drop faster and/or lower, by a pendulum effect. The thing is, the entire human world population circa 4000 BC was a few millions, which would include maybe a few dozens of thousands of magic users - with limited transportation and communication technology to disseminate magical knowledge. So it might be actually a mistake to placate data from that period onto the Shadowrun-era, which have a thousand times more magic users, plus coordinated efforts like the Ghost Dance to spread magical knowledge. Going the other way, the "First Age" might have been a period of magic that lasted from the very beginning of Humanity up until there were enough magic users (possibly as low as 1,000 worldwide, out of 100,000) to knock the pendulum off balance for the first time and start the cycle that last to this day.