Being overweight at the age of 50 may speed the onset of Alzheimer’s disease in old age, a study in the journal Molecular Psychiatry said Tuesday. A statistical comparison showed that every extra unit in body mass index (BMI, a height-to-weight ratio) in middle age corresponded to earlier onset of Alzheimer’s by about 6.5 months — what the authors termed a “robust” correlation. “A healthy BMI at midlife may delay the onset of AD,” the study paper said, referring to Alzheimer’s disease.