The outlook for people with dementia depends on the underlying cause , age at onset, presence and severity of other medical conditions, and the pace of decline in mental capabilities. Life expectancy can be as short as a few months or as long as 15 to 20 years.

Early-stage dementia caused by vitamin deficiency may resolve completely if caught and treated early. Dementia caused by a stroke (multi-infarct dementia) will often cause sudden deterioration in mental function that may stabilize until the next stroke. Dementia related to Alzheimer's disease, although incurable, can be treated with medications, which may slow the rate of decline. In most cases, however, the disease does progress gradually, and memory and functional abilities worsen.