Early-onset Alzheimer's is an uncommon form of dementia that strikes people younger than age 65. Of all the people who have Alzheimer's disease, about 5 percent develop symptoms before age 65. So if 4 million Americans have Alzheimer's, at least 200,000 people have the early-onset form of the disease. Most people with early-onset Alzheimer's develop symptoms of the disease in their 40s and 50s.
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