Dementia is a collective term used to describe various symptoms of cognitive decline, such as forgetfulness. It is a symptom of several underlying diseases and brain disorders. Dementia is not a single disease in itself, but a general term to describe symptoms of impairment in memory, communication, and thinking Generally speaking, the life expectancy of a person with dementia depends on the type of dementia they are diagnosed with, their age and health. Most studies seem to show that the average number of years someone will live with dementia after being diagnosed is around ten years.