At that point it's more likely she just made the number up in the usual fashion premodern cultures use to signify enormous magnitudes; complex life itself on Earth isn't even a billion years old, and even if it arose (much) earlier on their world - or the whole paradigm was different from the start, what with confirmed active divinities and all - it's unlikely any immortal bothered keeping track of years after the first millenia or two anyway. (People stop really paying attention to the matter after a few decades unless they have practical reasons to keep track of their exact age.)
Entirely plausible the dragons are outright biologically immortal, or as close as makes no difference, which would mesh well with their rivalry with the of the gods of their world - look no further than the assorted conflicts between different groups of supernatural immortals in various Earth myths for parallels.
However I don't think age is directly related to a dragon's "power level" (their world doesn't operate on D&D rules apparently) as evidenced in the confrontation between Tooru and her father, who going by his "Old Testament prophet" human form is a truly ancient being indeed - and pretty much admitted outright that if they actually fought he'd get his ass kicked.
IIRC it's been plainly mentioned Tooru's among the stronger dragons around, to the point that some of the others worry a bit about the implications to power balance if the conflict back home heats up (one gets the impression the tussle is at a low-intensity phase ATM). Ofc in the dodgeball chapter she mentioned in passing she's never "won" against Faf and Lucoa which makes you wonder how monstrously strong they are, and Elma can apparently more or less stalemate her in a serious duel...
As Kobayashi notes she knows some pretty dangerous beings.