Remember, several characters might have deleted at 400-550 hours, who might have lived much longer. Obviously someone who is going to delete his char at 650, does not get the chance to do so if he dies at 500. On the other hand, leader age skews this perspective a lot too. To really gauge a minimum for the average age death, you will need to also find out deleted characters who have fit into a proper range, let's say 350 hours and above. I agree though, that with a high amount of age deaths at around 500 hours, it's highly unlikely for the average to be over 600 hours.