Demons do not like to talk about their age in years, and it is not wise to ask, but in general the older they are the more powerful and higher in demonic society they have risen. All seven of the Demonic Masters date back to the time of the Fall, or shortly after. 

Occasionally a very old demon will descend from the heights of power and be reduced to a "common" demon, but through the application of favors owed and ancient knowledge they will quickly rise again. 

It is rumored that "ancient" demons differ from "modern" demons, but the exact method of demonic creation is unknown to outsiders.

-The Adventurers Guide to the Continent; Expanded Edition