Objective: To model the impact of dementia on past societies. Method: We consider multiple lines of evidence indicating elderly individuals to have been more common throughout the past than is frequently accepted. We then apply known dementia incidence/prevalence rates to plausible assumptions of past population structures to suggest prevalence in the past. Results: Dementia prevalence in premodern societies is likely to have been around 5% of the rate seen in modern, developed countries but with a total past incidence running into billions. Discussion: Dementia is often seen as a "modern" challenge that humans have not had to contend with before. We argue that this condition has had considerably greater effects than previously envisaged and is a challenge that humans have already withstood successfully, on one hand at a lower incidence but on the other without the considerable clinical, technological, and social advances that have been made in recent times.