'Downtime hunting', and whatnot, yeah.
All dates being approximate ...
2000 AD - 7000 AD : 6th World, Magic.
3000 BC - 2000 AD : 5th World, No Magic.
8000 BC - 3000 BC : 4th World (Earthdawn), Magic
13000 BC - 8000 BC : 3rd World, No Magic
18000 BC - 13000 BC : 2nd World, Magic
23000 BC - 18000 BC : 1st World, No Magic
Big Bang - 23000 BC : Who Knows?
So if Dunkelzahn was born roughly around 13,000 BC in the 2nd World, he probably lived just long enough to look around, maybe long enough to get to full adulthood, before he went into hibernation. Then, during the first part of the 4th World is likely when he achieved Great Dragon status, as he certainly had it in the latter part.
As for the hibernation, all the dragons (great or not) of whom their awakening from hibernation is written, with the sole exception of Ghostwalker, have emerged from lairs in their full-dragon physical form. (One might presume that too much magic would be required to keep them in humanoid form, and/or that their full draconic form can store more.) It is, perhaps, possible that these lairs are but areas whence they can emerge from astral space, but that's likely a matter for the GM to decide; maybe Ghostwalker's original lair-exit had been destroyed in the interim, and he had to find another way out. This does not, however, explain the 'downtime hunting' ...