TL;DR
The drug the longevity world bets on most is being put to its first real test, and the test subjects are pet dogs. Rapamycin, an old transplant drug, is one of the two candidates with the most evidence of slowing aging, and the Dog Aging Project's trial of it could report in as little as five years, around 2031. Dogs age faster than people and live in the same homes, so they compress a human-lifetime question into a few years. A yes in dogs would not put an anti-aging pill in a pharmacy; it would tell drugmakers whether a long, expensive human trial is worth paying for. For now no trial has shown rapamycin slows aging in anything as long-lived as a dog.
Below the line, for paid subscribers: The full story: where it stands, how we got here and what's dated next; our Odds board on 4 milestones, starting with "The Dog Aging Project reports results from its rapamycin trial by 2031"; and every source.