TL;DR
The weakness that comes with age may be less like a tire wearing thin and more like switches getting stuck, and scientists have started naming them. In fruit flies and older mice, a Singapore-led team found a gene, DEAF1, that rises with age and jams the way muscle cleans up after itself; turning it down restored strength, and exercise turns it down on its own. In mice, a Danish team found that aging muscle runs short of a fat that holds its power plants together; putting it back reversed the wasting. A named switch is a drug target, and one drug that acts on the second is already approved for a rare disease. For anyone who hopes to keep a young person's strength into a very long life, this is the first step: knowing exactly what to fix. None of it has yet been shown in people.
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 5 milestones, starting with "The planned one-year elamipretide trial in older adults enrolls its first participant by 2027"; our labeled speculation on what it adds up to; and every source.