In our case, DSS just lost his great grandfather, which triggered a heightened interest in the concept of death. He had been asking questions before, but not as frequently. We have realized that his issue age, like Amanda suggested. He couldn't vision the difference between us and his Pappy, and now Pappy was dead. That made him wonder when we would be dead. We are reinforcing the difference in ages between us and Pappy now, though that led to him asking questions about his grandparents, too.
It is a hard concept for kids, heck, it is a hard concept for adults, too.

