I don't know that there is a specific time when the concern about death "should" occur. I know it happens younger and more intensely for a lot of GT kids than for ND kids. I think it's a pretty common early side-effect of GT intensity.
You ask what you should do about your DS's questioning. Well, some of this is just trial and error and depends very much upon your child. I agree that you have to try to protect your child from truths that he's not emotionally ready to hear, but I also think that if you don't tell a child as much of the truth as s/he can understand, then you start to seem dishonest to the child, and trust can be damaged. That's the last thing you want!
I always figured there was a balance between truth and brutal honesty. I guess I usually started slowly, being gentle and emotionally protective of our DS. But if he kept prodding and questioning, I figured he was telling me that he wasn't getting the answers he needed. That's when we moved to the biological discussion. It troubled him a little bit, but not nearly as much as I feared it would. It actually seemed to make sense to him, and soon after he explored this answer, the questions stopped and he seemed happy again. He wasn't actively worried anymore.
We put as positive a spin on it as possible. We stressed the fact that if people and animals didn't die, there would be no resources for other people and animals to use for life. We stressed that a lot of the great advances of humanity probably came about because we feel our mortality, something that wouldn't happen if no one died. We stressed the fact that people don't hurt anymore when they die, don't see or feel anything anymore, since that seemed to be part of what was bothering him. (Which makes sense: if you don't understand death and you see someone being buried, you would worry about what they're feeling when they're underground.) And we stressed that love doesn't die.
Another thing we did was to give DS times that were appropriate for talking about death. This seemed to help with the inappropriate blurting out of questions and fears. But in the end, I think you just kind of have to ride that out. Young kids just aren't going to be appropriate about this!
So I guess what I'm saying is that maybe he's telling you he is ready for slightly more complete answers. You can wade into it slowly--you don't have to dive into it. But if he's not letting up on the topic, I think he's probably telling you that he's not buying what you've told him. He needs more. We found that giving our DS more complete answers made sense to him at last and let him let go of the issue.
You might ask specifically him what's bothering him about death. Maybe he's verbal enough to put it into words? You don't want to get into your own version of the joke:
"Mommy where do I come from?"
Mommy launches into a lengthy explanation of the birds and the bees, complete with drawings.
The child looked at her thoughfully, and then replied, "Oh. My friend Chris comes from Cleveland."
