B.D.W. - Before Demon War. Refers to any period of time before the resurrection of Ruby-Eye Shabranigdo in Rei Magnus.
A.D.W. - After Demon War. Refers to any period after the War of the Dark Lord's Descent.
The First Age (B.D.W. +5000 - A.D.W. 0)
This would cover everything from the general creation of the world all the way up to and including Ceiphied's and Shabby's climatic fight which resulted in: Shabby is "broken" into seven pieces, Ceiphied essentially dies (returns to the Sea of Chaos as primordial goo or something), and the huge crater in the middle of the formerly single continent becomes filled with water and becomes an ocean.
This would be the period of time where magical and technological wonders abound, nearly anything was possible though not necessarily proliferated amongst all of the races. This is the period of time where all of the other beings of the cosmology, like lesser gods, etc came into being from the excess 'cosmic goo' still floating around.
The Age of Man (B.D.W. 1000 - A.D.W. 1)
This is the period of time after the Fall of the Gods and up to (and including) the War of the Dark Lord's Descent. During this period of time, humans grew to be the dominant race among the three neutral races: humans, elves, and dwarves. The Ryuzoku remained apart from the rest of the world, devoted to their worship and the thwarting of the occasional Mazoku plot.
The Shinzoku, Aqualord, Firelord, Earthlord, and Airlord, were worshipped since their creation after Ceiphied's death. Seyruun becomes one of the prime Aqualord worshipping locations, at least among humans.
Humans learn the magic from the elves and the technology from the dwarves and proceed to advance the two fields quickly -- and perhaps even recklessly. As such, it was during this age that knowledge was interchanged and widely spread. Interracial relationships weren't very good, so wars were common; so common were the wars preceding the War of the Demon Lord's Descent, the ones the Mazoku manipulated and encouraged were not thought of as 'out of the ordinary.'
The Mazoku's plan was to awaken a piece of Ruby-Eye, so they ignited the wars between the lesser races to increase the amount of emotions floating around, so as to weaken the seal. They also attacked the Aqualord's temples at Dragon's Peak, taking down numerous Ryuzoku there in preparation of their major assault.
After the piece within Rei Magnus awakened and dubbed itself Demon King of the North, it proceeded to the Kataart Mountains, the lair of the Aqualord, with Chaos Dragon Gaav. The other Mazoku Lords situated themselves to form a seal around the area (the peninsula), cutting the it off from the rest of the world and, more especially, the Divine Power and other Shinzoku.
The Mazoku won in a sense, destroying the Shinzoku -- but it cost them dearly. Hellmaster Phibrizzo's two servants were ruined, Greater Beast's Priest Xelloss being the only one to walk away from the confrontation with the Ryuzoku. Gaav was killed but not ruined, his remaining essence sealed into a physical shell by the dying Aqualord Laguladia. The Demon King of the North faced a similar fate, doomed to remain within the Kataart Mountains for all (effective) eternity.
Much magic, technology, knowledge, and lives were lost because of the War of the Dark Lord's Descent though it is important to note that much of this happened before the true plan of the Mazoku came to fruition. The mortal races -- humans, elves, dwarves, and several other minor ones -- had been exhausted in resources, manpower, magic, and the like because of the devastating wars the Mazoku encouraged them to engage in.
The Age of Ruins
This is the period of history following the War of the Dark Lord's Descent. The Ryuzoku, again, retreated to their isolated temples and caves, and, this time, the elves and dwarves also retreated to their respective lands, all away from human enroachment.
Humans in the Demon Peninsula had to pick up the pieces again, hampered unknowingly by the other races(elves, dwarves, Ryuzoku, Mazoku) collecting and destroying knowledge that they deemed too dangerous for humans to have. Faith had to be restored to the people. Curative White Magic was either developed or put into much heavier usage than before because of the loss of Divine Magic. Technology, for the most part, was put on the shelf and forgotten.
The outside world went on with life as always though, on the Eastern Continent, magical knowledge was all but lost, leaving only the most simplest spells and techniques behind. There existed more remnants of the old technology out there, but people either stayed away from it or used it sparingly since they no longer knew how they work or how to repair them if they broke. Cannons, hand explosives, and primitive firearms were all but nonexistent.
There is absolutely no information about the Western Continent though -- considering that they were spared the effects of the Dark Lord's Descent -- there may be highly advanced civilizations there (relative to the Eastern Continent, anyway).
"Recent" History (A.D.W. 500 - p)
These are events which are spoken of and chronicled in the actual Slayers continuity. As of Rina's time, 200 years have passed.
over some time ago... - "Immortality Wars" occurred because of people fighting over supposed discoveries to immortality.((Sidenote: As of Slayers NEXT, the War of the Dark Lord's Descent took place 1012 years ago.))
over 320 years ago - The Magic Beast Zanaffar lays waste to Sairaag, the City of Magic. It was defeated by an unknown fighter wielding the Sword of Light. The Holy Tree Flagoon was planted to purify the poisons and evil left behind by the Zanaffar.
over 220 years ago - The ruler of Dirus Kingdom, King Dirus II, attacked the Demon King of the North (Stupid, isn't he? -_-). Dynast Grauscherra's army crushed the invasion, and Dirus II was sent back under the curse of the horrific Raugnaut Rasyavuna (a curse so powerful, only pure Mazoku of the upper heirarchy may use it).
over 205 years ago - The wife of the Crown Prince Philionel el de Seyruun is killed by an assassin. Said assassin was killed in turn by the elder daughter, Gracia Ul Naga Seyruun, who later leaves the palace and disappears.
over 200 years ago - Rezo-Shabranigdo is awakened and defeated, Chaos Dragon Gaav is ruined, Hellmaster Phibrizzo is ruined, Dynast Grauscherra is defeated, and the seal on the Demon Sea Penninsula is broken. A new era of exploration is, henceforth, unleashed.