More on the connection between diabetes and dementia. A paper published in the journal Diabetologia looked at HbA1c levels (a marker for diabetes) and diabetes status in relationship to cognitive status in large (N = 51,89) longitudinal study. Their findings:
Significant longitudinal associations between HbA1c levels, diabetes status and long-term cognitive decline were observed in this study.
I do have one concern about how this study is being reported in the media. For example, this piece in the Atlantic states
A high-carb diet, and the attendant high blood sugar, are associated with cognitive decline.
It should say “a high refined carb diet.” The distinction is important, simple sugars and refined carbohydrates should be avoided. Note that the same article recommends the Mediterranean Diet, a diet that actually includes a lot of unrefined carbohydrates. As the Harvard Medical Letter reports:
The carbohydrates in Mediterranean-style diets tend to come from unrefined, fiber-rich sources like whole wheat and beans.
peakmemory 
What I found amazing is the study controlled for so many other factors such as cholesterol, BMI, smoking, etc., etc. that could also affect cognitive ability. I would imagine there are few people that only have Type 2 diabetes. Frightening.