Board Thread:Lore Discussion/@comment-30529651-20161117203555/@comment-26795016-20170827111432

Pretty much what Blademaster said.

Saying that dragon souls increase lifespan is just a hypothesis without any evidence supporting it.

Miraak's immortality is most likely due to him spending his time in Apocrypha.

That being said if the Dragonborn chooses to become a vampire or serve Hermeaus Mora like Miraak did he may achieve some level of immortality.