What different stages of dementia are there?

Seven. Different type of dementias will have different stages and the time course for each stage will vary per individual patient. There are technically 7 stages including pre-dementia, but in clinical practice doctors really focus on mild, moderate, to severity of symptoms.
Good Q. Classically we think of mild cogn impairment (MCI), mild, moderate and severe. However there is also pre-MCI risk which is going to be critical in the efforts geared towards prevention.
There are several. There are a number of different staging systems. If you don't have a specific purpose (like fitting into a medication trial or looking for hospice) i find it useful to look at 3 stages. Mild dementia requires no outside assistance. People with moderate disease benefit from having others regularly check on them and assist with complex tasks. Those with severe disease require 24 hour supervision.