User blog comment:Legionaire22/How Horus turned his brothers/@comment-4608429-20120722201544/@comment-2253300-20120723221655

They can die of old age, it's just that if a Space Marine starts getting too old he will start looking for a beautiful death. No Space Marine could stand the dishonor of dying sick in some bed. He will look for death in battle before his body fully gives out on him.