User blog comment:Jopjopjop/Volume 19 Spoilers/@comment-25732759-20141120220802/@comment-1359322-20141121001606

Sirzechs and Ajuka actually all the FGS are post Maou Satan. Rias did mentioned that the Great War ended several hundred years ago, Sirzechs and the Ajuka are born about that time.

"“Long time ago, there was a three sided war between the Devils, the Fallen Angels, and God with its Angels followers. All three sides had a large army and they fought for almost eternity. As a result, all three sides lost most of their troops and the war finished several hundred years ago with no side winning.”"

- Volume 1 Life 2, Part 8