NOVEL NEXT Chapter : Time line
The Devil Civil War took place 500 years before the events of DxD when Sirzechs was 60 years old (Still not considered an 'adult' by Devil Standards). This means the war broke out in 1518 BC, which would make sense given how the Evil Piece System had to have been invented prior to 1535 (Heinrich Cornelius Agrippa's death).
With that in mind, we can confirm that Sirzechs & most likely the other New Satans were all born around 1458.
With that in mind, unless some new information comes out, this is the rough timeline of events I've managed to piece together:
100 BC - 100 AD: Events of the Ulster Cycle (Cu Chulainn died decades before 60 AD).
5th Century: Arthurian Lore. Ddraig & Albion sealed in the Boosted Gear & Divine Dividing.
1328 AD: Birth of Albedo, 15, August
1330 AD: Birth of MC (won't put the name well spoilers so), born 1, September
1458 AD: Birth of Sirzechs Gremory & the future New Satans.
1475-1500 AD: The Modern version of chess is created.
1518 AD: The Devil Civil War begins. (Sirzechs is 60).
1528 AD: End of Devl Civil War
At some point after this, the Evil Piece System is devised using Chess as a base.
c.1535: Historical death of Heinrich Cornelius Agrippa (Lord Gremory's Bishop).
2000 AD
April 9th: Birth of Rias Gremory.June 7th: Birth of Sona Sitri.July 21st: Birth of Himejima Akeno.
2001 AD
Feb 14th: Birth of Xenovia Quarta.April 16th: Birth of Hyoudo Issei.May 11th: Birth of Asia Argento.Sept 29th: Birth of Shidou Irina.
2002 AD
Nov 23rd: Birth of Koneko Toujou
2008 AD: Death of Cleria Bellial. The Shidou family move back to England.
2014 AD: Events of Slash/Dog.
2016 AD: Rias, Sona, Akeno & Tsubaki enrol at Kuoh Academy.
2018 AD: Events of DxD begin
Also as this is AU there will be many changes throughout the story being:
The improved fertility rate for all species (angles through the help of GOD's system) The presence of healing items (so phenex tears are more of a high-class or upper-class, medical item) More as the story goes on
Lucifer only created the 72 pillars
I doubt that the Maous or Lucifer created the Extra Demons as most of them considering that Mephisto is said to be more ancient than the first Devil spawned by Lilith coming from the same generation of the 4 original Maous.
The extra demon houses were present even before, the devils united under 4 Maous