Howard Tucker died on Dec. 22, 2025 at age 103, shortly after writing this essay for CNBC Make It. It is published here with his family's permission. I spent more than 75 years practicing as a neurologist, continuing until my hospital closed in 2022. Now, at 103 — and recently named the oldest doctor ever by Guinness World Records — people often ask how I stay happy, sharp and fulfilled. I don't believe there is one magic answer. Good genes and good luck certainly can give you a head start. But over the years, I've come to believe there are a few principles that matter a great deal. These are my three non-negotiables for a long and meaningful life — they are simple, and pretty much anyone can follow them. 1. Keep your mind engaged I tell all my patients that the mind is like any other muscle in the body: If you don't use it, it weakens. My work keeps me thinking, learning and solving interesting problems.…