User blog comment:Kogath/Monster Hunter: World Discussion/@comment-6050823-20180123003115/@comment-31029678-20180123134026

@WolfQueen Immortal no, eternal... kinda.

Elder Dragons   (Japanese  古龍種 )  are a class of monsters introduced in the first generation. This class is made up of rare, elusive monsters that have lived eternally since ancient times, that are able to bring destruction to whole ecosystems.

http://monsterhunter.wikia.com/wiki/User_blog:BannedLagiacrus/Elder_Dragon_Discussion

"lived eternally"... anyway like you said they can just be beings with  really long lifespans if not eternal, in both cases they surely have some special ability connected to their cells and their regeneration.