“A dragon can survive for centuries after reaching the great wyrm stage, but a dragon is mortal and cannot stave off death forever.”
Draconomicon v3.5, p. 15
In most campaign worlds, dragons who’ve reached the maximum age category, i.e. great wyrm, rank as some of the most powerful beings in the setting, a rank that they share with few other creatures. Only the most ancient of undead, elder titans, outsiders lords who sit at the top of their various planar hierarchies, and of course the gods themselves, can pose any real challenge to them, alongside the occasional mortal exemplar.
However, as the above quote makes clear, dragons are still beings with finite lifespans, and while they have alternative options available to them, such as guardianship (i.e. becoming a genius loci), undeath, or even trying for divine ascension, the default expectation is that they will eventually die. Insofar as the v3.5 iteration of D&D is concerned, this process is covered in detail in pages 14-17 of the 3.5 Draconomicon.
That same book puts forward an interesting discrepancy in this regard, though. On pages 99-100, it reprints (from the Epic Level Handbook) the rules for advanced dragons, central to which is the idea of “virtual age categories.”
Now, the operative word there is “virtual,” in that a dragon which has been advanced this way isn’t necessarily intended to connote that the dragon has advanced in age beyond the lifespan of its counterparts…but given that this is something which is only applied after it’s already reached great wyrm status, it certainly lends itself to that idea!
On a tangential note, I’ll confess myself to being partial to the idea put forward in The Immortals Handbook Epic Bestiary, Volume One, that each virtual age category a dragon has appends another “great” in front of its “great wyrm” designation. So a dragon that has two virtual age categories is a great great great wyrm.
But if we adopt the idea that these epic dragons (since the original presentation of virtual age categories in the ELH makes it clear that they’re a step beyond anything which could be called “normal” monsters) are indeed dragons who’ve simply kept on living, and growing, where their counterparts died of natural causes, we need to answer why that’s so. Why do some dragons continue to grow older and more powerful – presumably indefinitely – while others simply expire?
The v3.5 Draconomicon notes, in its section on the end of a dragon’s life, that a dragon who wishes to become a guardian must consume either 135,000 gp or 90% of its hoard, whichever is greater. In this regard, the book makes what I think is an insightful observation: that the hoard plays a key role in what happens as the dragon nears the end of its mortality. Rather than requiring it to consume its hoard, however, I’d posit that epic dragons need to have spent sufficient time sleeping atop a pile of sufficient treasure (nicely answering why dragons seem so intent on making beds out of coins, gems, and other valuables, and spending so much time napping on them).
Exactly how much time needs to be spent atop how much treasure is a variable I haven’t worked out, but in a very real way it doesn’t matter; the process requires a draconic lifetime to complete, so it’s not something the PCs or even the GM will ever need to work out over the course of a campaign. My personal benchmark is about three million hours’ worth of time sleeping (i.e. a little shy of four hundred years) atop one hundred fifty thousand gp worth of treasure, but that’s more of a placeholder than anything else (and probably needs to be adjusted upward, given the table for when great wyrms near the end of their lives on page 14 of the 3.5 Draconomicon).
A key aspect of this is that dragons don’t know that this is why they’re driven to sleep on treasure. That drive, which is propelling them toward immortality, is entirely subconscious. Even those few dragons who eventually figure it out have no desire to tell anyone; the good dragons don’t want evil dragons to find out, evil dragons don’t want more competitors, and neutral dragons are naturally self-absorbed even by draconic standards. So the greatest secret of dragons is one that remains secret even from dragons themselves. Even epic dragons may not know why they ascended where their fellows all died.
Or at least, that’s my take on squaring that particular circle. What do you think? Let me know in the comments!