From a Wall Street Journal story by Allysia Finley headlined “President Biden and the Science of Aging”:

There’s no shame in growing old. Everyone will eventually, though some will age more gracefully than others. Our 80-year-old president looks and acts every bit his age. Yet many people remain mentally agile and physically vigorous well into their 90s.

There’s something to the adage that age is only a number. Scientists increasingly are distinguishing between chronological and biological age. Someone can be biologically younger or older than his years on earth, depending on how his body and cells age. The best measure of that number comes from our telomeres—strands of DNA that cap chromosomes and protect genes.