Some academic work. There appears to be a correlation between B9 deficiency and dementia. After factoring in other variables.
https://www.medicalnewstoday.com/articles/low-folate-levels-…
...the researchers associated folate deficiency with a 68% higher risk of a dementia diagnosis.
Always a caveat:
Dr. Rotstein and her team also investigated the possibility that low folate levels could arise from, not before, preclinical dementia. This is hard to rule out due to the time it takes for cognitive decline to manifest.