My reflection on a brief read-through is that my Jan was bright as a child and as such was accepted for grammar school a year earlier than her peers. She gained a degree at Cardiff University and, throughout her life, until the onset of her dementia, she held challenging jobs.
Jan has young onset mixed dementia.
One case does not constitute a valid sample, but from posts by members here, there are many young onset families represented.
These people with young onset dementia will probably never be included in such research because they will never live to be in the target group.
While "Poor cognitive function is associated with the onset of dementia, disability, institutionalisation and death" may have some relevance elsewhere, not in our case, I think.
I should say there have previously been comments about dementia that have questioned whether it is the more highly educated people who tend to get it, and get it early.
I suspect that samples are simply not well enough selected.
As was said in the Pratchett programme - "it is random".