It is never too late to prevent Alzheimer’s disease. The trick is in stimulating your brain.
The Alzheimer's Association suggests the following to mainting your brain:

Feast on fruits and vegetables; a study on elderly men and women, found that those who drank fruit and veggie juice at least three times a week had a 75% lower risk of developing Alzheimer's than those who juiced up less than once a week.

Stay close to friends and family; in a study of 2.513 men, those with the least contact with friends and family in late life were nearly three times more likely to develop dementia than those with the most social actvity.

Get moving and exercise; in a study of 471 adult children of Alzheimer's patients, those who exercised regularly scored better on memory, problem-solving and other mental tasks than those who didn't exercise.