At least they do not know the exact reason, if they are even aware that they are dying. Just thinking of some war video footage and my own experience of the crash that broke my neck and back and almost killed me. And those that I have seen die. Some of this is a matter of mental state of awareness, or sudden onset of unknown factors to the individual, while others is a matter of an unseen assailant.
With some probability but not with certainty. At that age it could be anything.
The post isn’t about knowing in a meaningful or useful sense. In this context it is about the anxiety of the experience in the moments just before it happens.