There's no single standard classification used by everyone. Most physicians refer to three basic three stages of dementia, although they may be referred to by different names: 1) early stage or mild dementia, 2) mid-stage or moderate, and 3) late-to-end-stage, or severe. Some physicians and organizations, including the Alzheimer's Association, refer to seven stages of Alzheimer's disease, with stage 1 being "no impairment." Others refer to four stages of dementia or Alzeimer's: mild, moderate, moderately severe, and severe.
Whatever the stage names, because dementia of the Alzheimer's type is a progressive disease, everyone travels through its stages in the same order. The length of time a person spends in each stage is highly individualized. Some people travel through the disease from start to finish in three years, while others live 20 years or longer after an early-stage diagnosis.