Symptoms tend to develop slowly, often over several years. In the early stages of the disease, many people with mild dementia cope with just a small amount of support and care. As the disease progresses more care is usually needed. A 'typical' person with Alzheimer's disease takes about 8-10 years to go from the first signs of memory problems, to being severely affected, and ultimately to die due to frailty of body and mind.
However, the speed of progression of symptoms can vary greatly between different people.
Dementia is suspected by the typical symptoms which are usually reported by a relative or friend. A doctor may do a standard 'memory test' to confirm the diagnosis. However, some other conditions (sometimes treatable) have similar symptoms to dementia. Not all 'confusion' is due to dementia.
For example, depression in older people can sometimes cause memory problems. A person who reports a failing memory themselves, rather than a relative or friend, is just as likely to have depression as dementia. Depression is often treatable. Also, some physical problems such as thyroid disorders and brain tumours may cause 'confusion' in an older person and mimic dementia. Some tests may be done if symptoms are not typical, or if other conditions are suspected.