Stormonu
Legend
Anyone recalls from the top of his head how long a pit fiend aspirant had to spend in this special pit of fire? I think it was either 66 years, 666 years, 99 years or 999 years. If it's the last, then most pit fiends should be well over 1,000 years old just from having to have sit 999 years in this pit for their final transformation
Not off the top of my head, but from MC8 - Outer Planes Appendix:
Pit fiends are spawned from the powerful gelugons of the Nine Hells' eighth layer. When those icy fiends are found worthy they are cast into the Pit of Flame for 1,001 days (about 3 years) after which they emerge as a pit fiend (see Gelugon for more detail).
From the Gelugon entry:
Wholly unnatural creatures, gelugons can only be created by promotion from lower stations...First, the gelugon must have served flawlessly for 777 years. Any blemish on its record, even the slightest one, will eliminate it from the promotion list.
This means a pit fiend is at least 780 years old, not including the time it served to go up through the lower ranks. We're talking promotion of at least Mortal -> Larva -> Lemure -> Spinagon -> Abishai (Black -> Green -> Red) -> Barbazu /Hamataula -> Amnizu/Cornugon -> Gelugon -> Pit Fiend