It is well known that aging is the greatest risk factor for dementia development. In addition, other factors have been identified in epidemiological studies. However, families of people with dementia may have different views from the epidemiologically identified factors about dementia causes. This paper investigated dementia conceptions held by family caretakers of people with dementia in Japan. Forty caretakers were interviewed and asked why they thought their loved ones had developed dementia. In addition, 23 of the 40 caretakers were further asked two questions: whether they thought young-onset dementia and senile dementia were caused by the same factor and from what onset age they considered dementia to be senile. These two questions aimed to elucidate how aging factors affected their views on dementia onset. The important findings and implications can be summarized into three points. First, the findings suggest that for many family caretakers, the dementia causes in their loved ones cannot be dismissed as a physiological phenomenon of aging. Second, different explanations tended to be employed for causes between young-onset dementia and senile dementia. Senile dementia was often explained as an aging phenomenon, and it was more likely that its causes were attributed to controllable risk factors. On the other, young-onset dementia was often explained as an unpredictable misfortune and its causes tended to be separated from controllable risk factors. This difference indicates that people with senile dementia may be more subject to stigmatization. Third, the findings suggest that the age boundary between young-onset dementia and senile dementia is somewhere between the 6th and 8th decade of life.