Well, they can't be 3000 years old, because that's how old Aro is. And they were ruling before the Volturi.
The'd have to be around 4000- 6000 years old now.
Your supposition that they are older than the volturi based on the fact that they ruled first doesn't hold up with me.
I'm not saying that they aren't older, just that that reason doesn't hold water.
Perhaps they (stefan and vladimir) were "born" at the same time as the existing Volturi, but had gathered together a force worthy to rule prior to Aro's gathering his own force to challenge them.
They do say that for millennia they sat in their thrones and thought themselves gods, so perhaps that does signify that they are older, but as is put forth in many other scenarios in BD, and many of the other Twilight books, Vampires get sidetracked easily and many of them lose track of things so trivial as the passing of "mortal time". I'm just not sure
There's no sure way (as far as i have seen through my extensive "research")to prove that they were older than Aro. They merely had less time to "petrify", as they put it.
Which brings up an interesting point...
What do you suppose is the cause of the powdery translucence of the Volturi's skin? Presumably, it can't just be age, or even introspection, since there are surely other Vampires who have lived several millennia as well...
Is it just the nature of a Vampires psyche when he rules the world? That kind of fits, since the only ones in the series who have anything even close to that appearance are the Volturi and the Romanians, but that really isn't a very helpful theory... hmm...
Anyone got any ideas?