In this work, I explore and critically evaluate Aristotle’s views on the naturalness of dying from old age. His views are not straightforward, because Aristotle regards old age as a kind of decay and he talks about decay sometimes as natural and sometimes as unnatural. Nature, according to Aristotle, has two aspects, matter and form. I argue that, in Aristotle’s system, decay is always materially natural but formally unnatural. Likewise, natural death is death caused by old age and although getting old is never formally natural, it is, in a particular sense, materially natural. In Aristotle’s view, getting old does not have the same mechanism as inorganic decay such as wine’s turning into vinegar, but it is a result of the materially natural decay of lungs, which leads to the exhaustion of the vital heat in the heart.