Senile dementia is primarily a disorder of the elderly. Pre-senile dementia, or Alzheimer’s disease, is either the same or a similar disorder that occurs in someone under 60. It may be due to an underlying disorder such as hypothyroidism or a brain tumor , and in such cases it can sometimes be successfully treated if the cause is discovered in time. If this is not the case, it may be due to a progressive loss of brain cells, the cause of which is not known. Reasoning powers, memory, and other thought processes tend to deteriorate more quickly than they do in senile dementia, and the disease in a comparatively young person can be fatal within about five years.