A person in the prime of life who experiences memory loss and begins to lose the capacity to understand has what specialists describe as young-onset dementia.
It is estimated that about 40,000 people in Japan aged 65 or younger exhibit such symptoms, but beyond that little is known about them. In addition, 1.7 million elderly people have senile dementia, requiring nursing care.
There are about 70 ailments that trigger dementia, but four in particular stand out: Alzheimer's disease, dementia with Lewy bodies, vascular dementia and frontotemporal lobar degeneration.