Dementia in the elderly used to be rare, but why has it become a major social threat today? There can be many potential answers, but an ultimate one is clear: the longer life expectancy today. This knowledge indicates that "advanced aging" is a primary suspect in the origin of senile dementia. If so, then why can many elderly remain healthy at the same old age? We know, for example, that elderly people commonly have a certain degree of atherosclerosis and osteoporosis, but only some of them develop severe clinical symptoms at the same age. These different outcomes generally can be explained by "risk factors" in life (exercise, diet, individual background, etc). It thus appears to be a general pattern that advanced aging (after age 80) will set the stage for various senile disorders, but risk factors largely determine the onset age as well as individual specificity of their clinical manifestations. In this context, senile disorders including senile dementia would differ fundamentally from the pathogen-caused conventional diseases (AIDS, polio, cancer, Down's, etc.) by origin, incidence, and intervention strategy. This view would call into question the current definition of senile dementia as a conventional "disease" (Alzheimer's). The term "Alzheimer's disease" originally referred to "midlife" dementia, but it is defined today to be the same medical entity as senile dementia on the basis that they both display the same hallmarks and symptoms despite their onset age difference. Now, after in-depth scrutiny, we finally come to realize that they are not the same disease, but as different as heart failure at midlife versus the "same" failure at advanced age (i.e., a conventional disease versus a senile condition). Thus, by eliminating the age difference, the new definition has converted a senile condition into a conventional "disease", thereby changing the course of its scientific inquiry to miss the main targets. This may be why after extensive studies for 25 years, the origin of senile dementia has remained an enigma.