@daisytuliprose We spoke to ours about old age death at a similar age after their great grandfather passed. We said when people get older- maybe 100 years old or so (trying to keep it far away so as not to scare them), their bodies get old and worn out, like all things do eventually.
We talked about how wonderful it feels to go out for a special day, but how towards the end of the day we feel like we are done playing and we would like to leave now- and how when people become older they can feel the same way about their lives, that they have done everything and seen everything in their life they want to. That they feel tired a lot and like they are happy to go now, and then one day they slip peacefully away.
We talked about how noone knows what happens after death and some people believe we are born again as a new baby somewhere, or that we become part of the flowers and the trees, or that the dead go to heaven in the clouds and are watching over the people they loved.
We talked about how if there was no death they'd be no new life and no babies, or the Earth would be so full of all the people who ever lived they'd be no space to move. That its a cycle that allows for me and them to be born and have a turn on the Earth ourselves.
We talked about how when people are gone they are never really gone- as long as people remember them and talk about them they will stay with you always.
(I appreciate we've been fortunate that the only family deaths so far they have experienced have been great grandparents and elderly. But I think its helped to lay death out in a not scary way from around 4. They have never expressed worries about us or them dying, for example. It's also been a handy starting point for the 'are ghosts real' conversation which came a year or so later, as we could add it as another possibility for what some people think happens after death)