User blog comment:B214/Question and Answers 6/@comment-120.188.33.233-20160918121240/@comment-9595671-20160918190157

Wait, Dragons stop aging once they reach maturiy, then why isn't this mentioned on the Dragon page then? If this is true then it should that be listed there.

It's even said Evil Dragons in general can revive if their souls are intact, it's basically just a slower method of the Sephiroth Graal's ability. Hence why Azi Dahaka and Apophis implied they'll revive eventually after battles in Vol 21.

I think Nidhoggr's ability to revive is the best out of them all since he's claimed to have the power to survive Ragnarok. So even if Rias or Sirzechs destroyed his soul with their power of destruction, he might still be able to revive one day.