Just chiming in to let you know you are not alone. My son is 4 and we have been having discussions on death and not wanting to grow up for well over a year. We just approach things with a somewhat objective cycle of life perspective, paying careful attention to his feelings.
For what it is worth, I think for these types of kids it has more to do with their incredible attention to detail, awareness of the world around them, and ability to make connections than it does with a parent's own existential worries.
