Hello, dear readers, and welcome to the first column of high summer. You’ve kept our inbox full, so we’ll dive right in. — A recent column about vitamin D prompted a reader to ask this follow-up question: “It is my understanding — via a doctor — that when you reach 65 to 70, the body’s ability to make vitamin D begins to decrease. Is this true?” Yes, the doctor you spoke to is correct. As we age, …