When my friend Harriet Berne of Tenafly turned 90, she celebrated by investing in a 10-year CD. At 91 she endured a root canal and a year later enrolled in night school to brush up on her French. A bunion forced Harriet to see a doctor when she was 95, and the office staff was amazed to find it was the first time she used her Medicare card. Harriet’s only retort was “I believe in God, not doctors.”

Even though Harriet can no longer drive, she has her car tuned up every spring and starts it once a week, “in case of…