Older adults who regularly use the internet probably have more mental capacity.
It is possible to do either mindless or mentally stimulating things on the internet, and I doubt that just encouraging seniors to use the internet will reduce the risk of dementia. You could do a randomized-controlled trial where half of the subjects are given IPads or something similar and the other subjects are given the same amount of cash. I predict there will be no significant difference in outcomes.
Going by what I see on family Facebook posts, I'm not sure about this.