Died of old age?! Uhh... I know what REALLY happened! He actually died in the Dungeon that he was making for Finn. He was killed by the evil monster that was guarding the Demon Blood Sword. Nobody knew about this but it feels like I'm the only one who knows about it.
Don't believe me? If you watch the Dad's Dungeon episode and you make it to the point where Finn and Jake made it to the final room where the sword was, the pitched dark room has bones in that room and that's Joshua! It makes since because all of his last recordings and his video camera was in that SAME room!