What term describes the act of restoring someone or something to youthful vigor or appearance?

The term that most accurately describes the act of restoring someone or something to youthful vigor or appearance is "rejuvenate." This word specifically implies a return to a state of youthfulness or freshness, evoking imagery of vitality and energy that is typically associated with youth.

In contrast, while "restore" and "renew" also suggest improving or bringing something back to a former state, they lack the specific connotation of youthfulness that "rejuvenate" encompasses. "Revitalize" suggests infusing new life or energy into something, but again, it does not specifically imply a return to a youthful state as effectively as "rejuvenate" does. Therefore, "rejuvenate" is the most precise choice for conveying the idea of restoring youthfulness.