This explains the various patterns of armour and any further discussion is merely speculation without source quotation.
Now the question to be asked is 'when did a heretic become a heretic?'. If using the Ancient Warrior talent the chaos marine could be as old as the HH itself. There are countless hundreds of characters in the fiction that were merely marines at the time of the HH but have since risen to places of power in their chapters and legions, and these would clearly have to use the Ancient Warrior talent to represent this. Armour wise, i believe a character who takes Ancient Warrior would reasonably have been issued with marks IV through VII because whilst any marine can live for many centuries the vagaries of warp travel can accentuate their longevity.
I personally don't buy into this "i was on my way to the horus heresy when i got tossed about in a warp storm and now it's 10,000 years later but i'm only a month older than i was at the outset of the HH!" malarky that seems to be being thrown around a lot lately. If you want to play it that way, you wouldn't get access to Ancient Warrior or the other talent that allows an Infamy point expenditure for GM guidance (something like Wisdom of the Ancients?).
Any chaos marine not taking Ancient Warrior would thus in my mind have had access to marks VI through VIII depending on how recently they fell from grace and which chapter they were part of.
One of the problems of trying to come up with a single mechanic to determine which marks each character can start with is there are some glaringly obvious exceptions. For example in Deathwatch, Rites of Battle adds a table to roll to determine starting mark, but the Consecrators chapter (Dark angels founding that have lots of ancient relics) have no modifiers to the table, despite not using any newer marks of armour. Similarly in First Founding the Raven Guard do not automatically get Mk VI armour.
I'd hope that ultimately when we get the 'players handbook' for Black Crusade that it'll feature as full a section on armour as Rites of Battle did for Deathwatch and that most GMs use their imagination and common sense in doling out power marks of armour to their players