In Old Age Simone de Beauvoir argues that Plato and Aristotle reached opposite conclusions concerning old age: the optimistic and the pessimistic. B.Y.L. Simon challenges Beauvoir's suggestion on three main points, claiming: (1) Plato and Aristotle both agree that the elder is to rule and the younger to be ruled and that people must respect elder persons and old parents. (2) They do not appreciate old age in general but only exceptional cases of philosophers or the elite. (3) They think that old age is going to degenerate not only physical conditions but also intellectual capacity of the soul. In this paper I examine these three points and show that they are not supported from the texts of Plato and Aristotle. First, I point out that their physiological theories of old age are significantly different In Tim. 81B-D Plato explains the physiological mechanism of old age based on the assumption that connate elemental triangles, which function to digest nutrients, are doomed to wear out with age. However, he never suggests old age necessarily brings about deterioration of intellectual faculties. Plato distinguishes old age from disease and then suggests a way of living a healthy life to harmonize movement of soul and body. His suggestion is not limited to the intelligent elite but targets ordinary people (Tim, 88C). On the other hand, Aristotle believes that ageing leads to deterioration of the intellect. According to his physiological theory Vital Heat, which plays an essential role to cook down food into blood carrying necessary nutrients to each part of body, will be decreased as the process of ageing hardens the lungs and destroys the balance of its cooling mechanism. He supposes that decreasing or lack of the amount of Vital Heat would make the blood colder and scarce and eventually could cause degeneration of sense perception and intellectual ability. Moreover, he does not make a clear distinction between old age and disease and even approves of old age being called a natural disease in GA. 784b Second, I investigate the usage of terms used to refer to old age or the elderly in Plato and Aristotle. There are two major groups of words to indicate old age in Greek: πρεσβυσζ and γηραζ. Γηραζ means old age in a proper sense but πρεσβυζ indicates relatively elder one and has a positive meaning of importance or ambassador. Plato uses πρεσβυζ twice more than γηραζ. On the contrary, Aristotle applies γηραζ twice more than πρεσβυζ. Plato seldom uses these words in a negative way although he sometimes ironically refers to old age. Aristotle uses γηραζ more often due to his greater interest in biological research. He observes a large variety of species whose lifecycles are much shorter than human beings and tries to identify specific features in old age to each creature. Various unpleasant phenomena of ageing are objectively explained with only two principles: dryness and coldness. Their contrasting usages of these words suggest that their attitudes toward old age are quite different. Third, Aristotle certainly agrees that the elder or full-grown adult is to rule the younger, but he criticises the office of Elders in Sparta as a gerontocracy in Pol. 1270b-1271a because he believes that there is old age of mind as well as of body. By contrast, Plato allows the examiners of the magistrates to hold their important office until the age of 75 in Lg. 946C. Finally, I indicate that Plato gives far more sympathetic treatment to very old grandparents or parents than Aristotle.