What part of the brain or nervous system it affects dementia?

Dementia is caused by brain disease or injury so it obviously affects the brain

Dementia is not a disease but it is simply the affect of strokes, Alzheimer, and brain damage. To be specific, it affects the memories, and the ability to function well in everyday life.

As you get older, it is even more likely to get this.

it mainly affects the back region of the brain and the front. where your motor skills are vulnerable.