User blog comment:B214/Question and Answers 6/@comment-120.188.33.233-20160918121240/@comment-29963364-20160918171053

1. Dragons have infinite life-spans. Once they reach maturity, they stay alive untill they are killed.

2. Nidhogg was indead using Phenex Tears to regenerate himself during his fight with the ORC, but he himself already has immense regenerative abilities. He just wanted "extra insurance" against Issei and his friends. All Evil Dragons (except Crom Cruach) were enhanced by the Sephiroth Graal, and Issei and his friends still managed to kill most of them.

3. In real mythology, and in the DxD series, Nidhogg is a dragon that has nawed at the roots of Ygdrassil for thousands of years. And due to this, along with his own immense greed, he has gained the ability to revive himself after death. Meaning that even if he's killed, he will come back to life. Though it will take a very long time for him to do, and he'll likely need to naw from the Ygdrassil roots to regain his strength after he does. This can probably be countered by sealing or destroying his soul.