I mean seriously..it wasn't old age that killed him obviously. Could he have died from a disease (heart disease like Goku would be interesting)? I doubt he was killed by another shinobi since he was so powerful..I highly doubt it was suicide too because it seemed like he really wanted to protect the village and its people.
its probably some funny incident....like he slipped on the toilet, broke his neck and died...or sth like that
seriously...its probably something funny, considering his character and nobody talks about some heroic deed of his before dying....like all the other hokages did
He died fighting in a war. It is presumable he was fighting many many (like thousands) of enemies and finally died, due to his old age finally getting to him and the likely fatigue from fighting from hours or days on end.