My son (4.5) started talking about how he didn't want to die about six months ago. The conversations have stayed with just my husband and I, so no one has noticed.
Ever since he was about two, I've talked about death as we came upon it, such as a bug he squished, with a matter-of-fact approach hoping to avoid excessive fear. I don't know if that will work for us or not. DS goes through phases of talking about it and each time comes back with new questions, so I know it's an ongoing topic for thought.
We have a very old cat (22) who is not long for this world. I've felt it was important to prepare my son that one day he will be dead, so I've discussed how the cat has had a very long life and is now sick and how he needs to love him up now while he is still here. DS has said he will miss his cat when he dies, but hasn't been overly emotional about the thought.
I dread the reality of that day, though. I don't know how he will handle the death of something he loves.