Lifestyle changes such as regular brisk walks and eating more vegetables, fruit and oily fish are known to be protective.
The study involved more than 21,000 men and women across the USA.
Regular visits to the dentist may help prevent dementia.
The study offers hope.
A study of around 5,000 older people found those with the condition were 35 percent more likely to develop dementia.
Keeping the brain active helps store healthy neurons - improving thinking skills as you get older.
The study found that sustained low pay is associated with "significantly faster" memory decline.
"Our idea was people with a rapidly declining sense of smell over time would be in worse shape."
Consuming just seven units of alcohol a week - half the recommended maximum - fuels this.
Medications containing CBN could work without causing the individual to become 'high.'