Restoring “youthful properties to old cells sounds like magic,” says Thomas Rando, director of the Broad Stem Cell Research Center at University of California at Los Angeles. “But it happens all the time in biology. We just haven’t been able to capture it therapeutically.”

When a sperm and egg from two people who are decades old combine, the resulting cells don’t inherit the age of the parents’ cells. “Every time there is a fertilization event, the clock of aging is set back to zero,” says Dr. Rando.

Resetting a cell’s age to zero would erase its identity. Dr. Rando and other researchers are aiming for “partial reprogramming,” in which, say, an old muscle cell or a neuron becomes a younger version of itself. Early laboratory studies are encouraging, Dr. Rando says, but the approach is far from being tested in humans.