The stages of dementia can vary depending on the individual and the root causes of the dementia, notes Mayo Clinic. On average, people live about 4.5 years after being diagnosed with dementia, reports WebMD.
Alzheimer's disease often progresses in a more steady way that can make it difficult to identify the exact stages, states the Alzheimer Society of Ireland. Vascular dementia, another common type of dementia caused by small strokes that affect the brain's ability to function, often has more clear indications between the stages. This is because the decline generally only happens after another stroke occurs. Lewybody dementia can be even more difficult to determine because the patient's abilities often change drastically from day to day. This type of dementia often lasts for six to 12 years before death occurs.
Alzheimer's disease can be more predictable, but it still varies wildly depending on the individual circumstances, explains HelpGuide.org. Stage one, or mild cognitive impairment, may last two to four years. Moderate Alzheimer's often lasts two to 10 years, while the third and most severe stage lasts one to three years or more. Alzheimer's disease also has a preclinical phase, in which the brain changes but there are no noticeable symptoms. This stage can last as long as 20 years, according to Mayo Clinic.