Superman is an American icon and can never die. Every story where it has happened always resulted with him coming back to life.
Due to his super fitness- it is logical that he would live a lot longer than an average human. His body resists incredible physical damage, but it is not completely indestructible. He can bleed and he does age throughout time. He has a super powered body which requires a super powered heart in order for it to run. One day that heart might not be be able to maintain his super powered body and what he puts it through any more than an average man's heart that runs an average man's body.