User blog comment:B214/Volume 22 spoilers/@comment-201.215.165.7-20170112030400/@comment-30924344-20170112154607

A dragon can't die due to old age and the same goes for Ophis and Great Red. Issei of course masters to maintain his life force at infnite in the future and as it is infinite, so is his life force because his body is made from the flesh of a dragon and so the limtless aging of a dragons body applies to him.