Can radiation treatment for skin cancer on the top of the head of a 94 year old woman cause dementia?

It's difficult to answer this question because most people around this age already have senile dementia, more or less, and the severity can dramatically change in days or weeks.

Very unlikely. But depends on dose. Discuss with team. And would come on suddenly not over years.

Highly unlikely, as the more reasonable etiology is Alzheimer's disease, vascular dementia, or a combination of both. But should exclude, thyroid problems, vitamin deficiencies, especially B-12, medication side effects, etc.