I started talking about death when they asked questions so about 3 years old. They wanted to know about their daddy's mum who had died of cancer more than 25 years ago. I kept it age appropriate (I think) and they do understand that we all die sometime but we try to be as healthy as we can so it is not for a long time.

My eldest really struggled with it last year and we had to rehash it again to give him some answers. He was able to understand more and that helped.