No, not the BioWare series that has untapped potential. I'm talking about Elder and Ancient dragons (don't have Dawnguard or Dragonborn, so those two are my highest). Since all dragons are immortal, and none were "born", but rather created by Akatosh, how are there "Elder" and "Ancient" dragons? My theory is that they're the dragons who go the longest without being brought down in battle, and the ones who have won the most battles. When Alduin revives them, they get plopped back down to regular-level dragons (or Blood/Frost), and have to regain their full power (kind of like in Minecraft, where you lose experience when you die).
Spoiler Miraak proved himself by showing his power. he was a Dragon Priest but it still seems legit. Most of his leutenients(THE ACTUAL DRAGONS) were stronger then average aswel. Maybe it has a bit to do with dominacy or perhaps Elder and Ancent are only age based, thus gaining more power through the decades
I always assumed they proved themselves to Alduin. I mean yes, they cannot die perse, excluding the Dragonborn(I was refering to his/her power), however, they can be brought down.