According to the medical journal Neurology, staying active cuts the risk of Alzheimer's disease by 50% at any age. And this doesn't mean you have to spend hours in a gym exercising. Things as simple as housework, taking walks, etc count.
Personal note: My husband has COPD and Alzheimer's. He was a very active person most of his life. When his COPD really slowed him down his Alzheimer's because worse.