A quick look and I see that it's not very common there at all. You have very different causes of mortality to western countries. Something we would have seen more maybe a hundred years ago or more. Hopefully as you get more western processed foods you don't also get the western chronic diseases.

Here's a link for a description of dementia. It's a complex disease that doesn't really have a quick explanation and has an array of symptoms.
https://www.nia.nih.gov/health/alzheimers-and-dementia/what-dementia-symptoms-types-and-diagnosis