The authors of a recent article in The Lancet found nine “potentially modifiable” risk factors that appear to account for 35% of dementia:

Our results suggest that around 35% of dementia is attributable to a combination of the following nine risk factors: education to a maximum of age 11–12 years, midlife hypertension, midlife obesity, hearing loss, late-life depression, diabetes, physical inactivity, smoking, and social isolation.

I suspect that genetic factors may affect these risk factors as well, so making behavior changes focused on  these factors may have a limited effect on dementia risk (the authors themselves acknowledge that “[t]he available evidence for the effect of lifestyle changes on cognitive decline is mixed”). But since changing behavior to avoid these nine risk factors is probably a good idea anyway, it’s good to know it might reduce dementia risk too.