New research suggests that Americans’ risk for developing dementia over a lifetime may be higher than previously thought. A study found that after age 55, people have up to a 4 in 10 chance of eventually developing dementia — if they live long enough. The risk is highest after age 75, but doctors say there are ways to reduce that risk. Controlling blood pressure, diabetes and cholesterol is good for both your brain and your heart. They also urge staying physically and socially active. The study was published Monday in Nature Medicine.