SEATTLE — Ever since last summer, when Lynn Gemmell’s dog, Bela, was inducted into the trial of a drug that has been shown to significantly lengthen the lives of laboratory mice, she has been the object of intense scrutiny among dog park regulars.

To those who insist that Bela, 8, has turned back into a puppy — “Look how fast she’s getting that ball!” — Ms. Gemmell has tried to turn a deaf ear. Bela, a Border collie-Australian shepherd mix, may have been given a placebo, for one thing.

The drug, rapamycin, which improved heart health and appeared to delay the onset of some diseases in older mice, may not work the same magic in dogs, for another. There is also a chance it could do more harm than good. “This is just to look for side effects, in dogs,” Ms. Gemmell told Bela’s many well-wishers.

Technically that is true. But the trial also represents a new frontier in testing a proposition for improving human health: Rather than only seeking treatments for the individual maladies that come with age, we might do better to target the biology that underlies aging itself.