From the internets:
"In the mythology of the Aztecs, the first age of mankind ended with the animals devouring humans. The second age was finished by wind, the third by fire, and the fourth by water. The present fifth epoch is called Nahui-Olin (Sun of Earthquake), which began in 3113 BC and will end on December 24, 2011. It will be the last destruction of human existence on Earth. The date coincides closely with that determined by the brothers McKenna in The Invisible Landscape as “the end of history” indicated by their computer analysis of the ancient Chinese oracle-calendar, the I Ching."
"Archaeologists claim that the Maya began counting time as of year 3114 B.C.
This is called the zero year and is likened to January 1, 1 AD. All dates in the Long Count begin there, so the date of the beginning of this time cycle is written 0-0-0-0-0. 13 cycles of 394 years will have passed before the next cycle begins, which is in year 2012 A.D. (13-0-0-0-0)."
"The Mayas overcame this problem by using a third dating system which enabled them to identify a day uniquely within a period of 1,872,000 days - approximately 5,125.36 solar years."
So about 5,125.36 years is a World, and the Fifth world began about 3114 B.C. Presumably the Fourth ended about then.

Source:
http://www.world-mysteries.com/sar_3.htm#top (The most non-esoteric of the lot I found)