Dementia is a set of symptoms that can be caused by a number of different disorders. Impairments in thought, communication, and memory are all indications of dementia. Alzheimer's disease is more common in older individuals, although it can also strike persons in their 30s and 40s. Early-onset (or younger-onset) Alzheimer disease is defined as Alzheimer disease that develops before the age of 65. The early-onset form of Alzheimer's disease affects just a small percentage of Alzheimer's patients. When the disease strikes, many of them are in their 40s and 50s. Different kinds of dementia have different effects on people, and each person will have their own set of symptoms. Two or more of these symptoms must be present in order for a person to be diagnosed, and the symptoms must be severe enough to interfere with everyday life.