The overwhelming factor emerging from genetic studies of the dementias and other central nervous system neurodegenerative conditions is abnormalities of protein handling.

This age cutoff is defining, as similar sets of symptoms due to organic brain syndrome or dysfunction, are given different names in populations younger than adult. Up to the end of the nineteenth century, dementia was a much broader clinical concept. Well into the second half of the twentieth century, dementia of the elderly was called senile dementia or senility and viewed as a normal aspect of growing old rather than as being caused by any specific diseases, while Alzheimer's disease was seen as a rare disease of middle age, until the neurologist Robert Katzmann signaled a link between "senile dementia" and Alzheimer's.

Dementia is a non-specific illness syndrome (set of signs and symptoms) in which affected areas of cognition may be memory, attention, language, and problem solving. It is normally required to be present for at least 6 months to be diagnosed; cognitive dysfunction that has been seen only over shorter times, in particular less than weeks, must be termed delirium. In all types of general cognitive dysfunction, higher mental functions are affected first in the process.

Especially in the later stages of the condition, affected persons may be disoriented in time (not knowing what day of the week, day of the month, or even what year it is), in place (not knowing where they are), and in person (not knowing who they are or others around them). Dementia, though often treatable to some degree, is usually due to causes that are progressive and incurable.[citation needed]

Symptoms of dementia can be classified as either reversible or irreversible, depending upon the etiology of the disease. Less than 10% of cases of dementia are due to causes that may presently be reversed with treatment. Causes include many different specific disease processes, in the same way that symptoms of organ dysfunction such as shortness of breath, jaundice, or pain are attributable to many etiologies.

Without careful assessment of history, the short-term syndrome of delirium (often lasting days to weeks) can easily be confused with dementia, because they have all symptoms in common, save duration, and the fact that delirium is often associated with over-activity of the sympathetic nervous system.[citation needed] Some mental illnesses, including depression and psychosis, may also produce symptoms that must be differentiated from both delirium and dementia.

Chronic use of substances such as alcohol can also predispose the patient to cognitive changes suggestive of dementia, although moderate intake may have a protective effect.

Dementia is not merely a problem of memory. Additional mental and behavioral problems often affect people who have dementia, and may influence quality of life, caregivers, and the need for institutionalization.

Depression affects 20–30% of people who have dementia, and about 20% have anxiety. Psychosis (often delusions of persecution) and agitation/aggression also often accompany dementia. Each of these needs to be assessed and treated independent of the underlying dementia.

The Canadian Medical Association Journal has reported that driving with dementia could lead to severe injury or even death to self and others. Doctors should advise appropriate testing on when to quit driving.