Probably the scariest thing for a child is the death of a parent. If there has been a death recently that has affected your family, or if your child asks you earnestly about death, you can almost assume that underlying it is the concern that you might die.
photo by Stuart Miles, http://www.freedigitalphoto.net
Here a blanket reassurance is in order. You can state that you will not die until your child is all grown up and (if you're lucky) has children of his own. Then, when you are an old, old grandparent, it will be time for you to die. Most children are comforted by having this terrifying event put off into an unimaginable distant future. They know they aren't grown-ups yet, so they don't have to fear your dying. Although you can't be absolutely certain that you will live that long, this is not the time to go into all of the possibilities. Your confident promise to stay alive is the reassurance your child need.