Well these account for 35% of your risk. Is exercise really worth 3%

Nine factors that contribute to the risk of dementia
Mid-life hearing loss - responsible for 9% of the risk
Failing to complete secondary education - 8%
Smoking - 5%
Failing to seek early treatment for depression - 4%
Physical inactivity - 3%
Social isolation - 2%
High blood pressure - 2%
Obesity - 1%
Type 2 diabetes - 1%

These risk factors - which are described as potentially modifiable - add up to 35%.

The other 65% of dementia risk is thought to be potentially non-modifiable.
Presume that means genetic!