Yeah, the Cycles of Magic doc has Erhan stating Atlantis sinking (3114 BC) marked the end of the 4th Age, beginning of 5th, and that the start of the 6th Age was on 2011 (the Maya were a year off). So that gives the 5th cycle a length of 5125 years.
The big assumptions are that the whole flowery "Cycles of Magic" title is actually a close fit to a sin wave and that each Age is roughly equal in length (like maybe +/- x years).
Though having fun with those assumptions, since the total length of an age is 5125 years, then the peak/trough of each magic cycle happens at about 2562.5 (rounding to 2563) years after the cycle begins.
So setting the start of the first age at 0, you have:
0 - 23,614 BC - 1st Age start
2563 - 21,052 BC - 1st Age trough
5125 - 18,489 BC - 1st Age end/2nd Age start
7688 - 15,927 BC - 2nd Age peak
10,250 - 13,364 BC - 2nd Age end/3rd Age start
12,813 - 10,802 BC - 3rd Age trough
15,375 - 8239 BC - 3rd Age end/4th Age start (Earthdawn age)
17,938 - 5677 BC - 4th Age peak
20,500 - 3114 BC - 4th Age end/5th Age start
23,063 - 552 BC - 5th Age trough
25,625 - 2011 AD - 5th Age end/6th Age start
Now if I remember correctly, in Earthdawn everyone went underground to avoid the Horrors for 400 years. Averaging it to an even split (200 years on either side of the peak) this means that the Horrors started coming in earnest around 5477 BC and dwindled low by 5877 BC.
So here's where my math diverges from FastJack, I get the peak of the 6th Age at 4574 AD, with the Horror-Hoedown starting around 4374 AD (assuming 200 years on either side of the peak).
Again, the dates are all based on the Cycle of Magic document. And there's a good helping of assumptions.
Fun things to add as flavor in campaigns...
Earth's precession (
http://en.wikipedia.org/wiki/Precession_%28astronomy%29#Astronomy) is approximately 26,000 years...about the time from the start of the 1st Age to the end of the 5th. Making the 6th "special" ... how you want to define special of course is up to you; Horrors come early and often, new Age of Dragons, rise of the Jackalopes, etc. Anyway, the "Great Cycle of the Precession passing through 5 Ages and starting over again" is one of those nice semi-anchored in reality plot devices at your disposal.
As FastJack pointed out, with modern humans emerging around 50,000 years ago, that's about two Precession cycles or 10 Ages, with the 6th Age being the 11th Age from then.
Anyway, love the Cycle of Magic doc for the info it plays with. Lots of fun to play with the cycles, fitting mana spikes to it. Like the "mini-awakening" which happened in the Middle Ages in Europe (I think that's from the old Germany Sourcebook, or maybe an adventure. If I remember I'll check when I get home.). With the spike of Kabbalah, Alchemy, Goetic magic, and Hermeticism, all really taking off during that time period you can incorporate Dr. Dee's Enochian language as an actual language/script of spirits, Kabbalah Tree of Life as an initiate's path and so on.