Search: youngest alzheimer's
Why: I don't remember... lulz.
Answer: 27! What!
It is very rare to show signs of dementia before age 40, however the youngest person ever diagnosed with Alzheimer's was 27 years of age. Alzheimer's and Down's Syndrome are both on chromosome 21, and patients who have Down's Syndrome invariably develop Alzheimer's if they live long enough.I also see a few notes that say "as early as 16 years of age," but no real sources, so let's ignore them.
Any Alzheimer's diagnosed before the age of 65 is called early-onset Alzheimer's disease, but it makes up only 5-10% of sufferers (in 2006, there were 26.6 million people with the disease worldwide).
Source: Aging Care, CBS, Wikipedia
The More You Know: In 1906, the disease was first described by German psychiatrist and neuropathologist Alois Alzheimer as "presenile dementia."