Because, when you come back to Gantz the same way you entered. Or that other theory that says, it makes a copy of you right before you get injured. If that is the case, if his organs are all aged right before he gets injured, then that's how he's comming back... now my question..,
If when they are injured.. kei almost dead.. old man's legs gone.. but when the mission is over, they are brought into the other room full and as good as new. Why is Esp1's body aging like an old man.. I can understand if that was before his death, but then., shouldn't it never get worse?
And because its a natural thing not caused by Gantz. If that was the case, Gantz could make all the old guys back in their prime or all the kids in their prime.