You have had some very good answers, so good that I have nothing to add to them on their own terms. As several of them rightly point out, it is a question of semantics. The question as it was posed is too ambiguous to be meaningful.
By way of analogy, suppose someone asked you as a physicist when a material is at its strongest? It sounds perfectly simple, until one stops to ask oneself what is being asked. In fact if a layman were to ask you that and you began to explain why you could not answer it meaningfully off the cuff, he would surely accuse you of obstructive doubletalk. The question obviously is perfectly obvious.
Your insisting on raising questions of temperatures, hardness, toughness, work of fracture, fibre strength versus bulk strength, tensile strength versus compressive or shear strength, you name it, is patently malicious obtuseness!
So, if dying is the process of becoming less alive, then from some points of view we are constantly dying every time our skin sheds a new cell into our epidermis or an erythrocyte sheds its nucleus. And as someone pointed out, the dying of cells on a large scale is a vital component of the process of our metamorphosis, which begins quite early in embryogenesis, and does not meaningfully stop before senile decay sets in.
Broadly speaking it is simple to choose a point before which we cannot usefully be said to be alive, and it certainly is easy to agree to within a few minutes at most when someone dies from having been comprehensively squashed by a falling boulder, but whether we can define "living" and "dying" for the ordinary man in the street or in a hospital bed, is open to discussion, and not necessarily useful discussion.
One thing I do guarantee you though, is that many alternative and hypothetically rational definitions are possible, if not necessarily profitable out of context.
