Alzheimer’s disease is defined as dementia that develops in old age, and by old age we mean people over the age of 60; if it develops before the age of 60, it can be defined as early-onset dementia. It is not impossible as to whether dementia can occur in young people because there are more causes of dementia, such as the common vascular dementia. If cerebrovascular disease occurs in young people and affects the cognitive function of the patient, dementia may also occur. Dementia caused by other factors, such as metabolic or common hypothyroidism, can also cause cognitive impairment. Another example is that nowadays more people drink alcohol, especially alcohol abuse can affect the metabolism of vitamin B12 and vitamin B1, and may also develop Wernicke-Korsakoff encephalopathy and brain atrophy. Cognitive impairment can also occur in young people who drink too much alcohol, due to metabolic encephalopathy due to alcohol, including dementia. For some infectious factors, such as leptospirosis, syphilis, and HIV infection, it is possible to affect the brain and develop brain lesions, which can lead to cognitive impairment. Most dementias are acquired, but there are also congenital conditions called congenital stupidity, also known as trisomy 21, in which the symptoms of dementia are present at birth. For example, people with congenital stupidity also suffer from cognitive deficits, including poorer than normal memory, numeracy, learning ability, and analytical and synthesizing skills.