Dementia, at least of Alzheimer's type, starts at least 20 years before symptom onset and is multifactorial, so much that no single factor plays a role ("explains the variability") more than 5%. Employment, as almost all other factors, like having family/kids, higher education, wealth etc etc, might be a correlative or a confounding variable, depending on the study design because the disease mechanisms are so long and convoluted.

In the egg and chicken dilemma, I believe that the cognitive decline causes the social inactivity and not the reverse. Get your retirement because that will not cause your dementia.