Just a random thought that crossed my mind.
I know marines live far beyond normal human years (with 200 years being "young") and some
marines, particularly in dreadnoughtsare very long (isn't Bjorn a Heresy Veteran?).
But assuming they survive every battlefield encounter, and never get Dreadnought interred do
they eventually succumb to age?
And by the same logic, as humans age we get weaker. Is this the case for superhumans as well?