User blog comment:FairyRaven/Fairy Tail Chapter 465: 400 Years, Review and Commentary/@comment-71.222.52.171-20151208025611

The time travel of the dragon slayers does explain why Natsu is alive in modern times as a demon he would have been immortal but since he was seperated from his tome (by extension his powers) he should have lost his immortality and well died from either old age, fighting, sickness, ect.