There is epigenetic age reset that happens during the conception that "resets" the cells to their young version like kind of a factory reset that cleans up the aging marks and other offsets that have happened during life so they don't get transmitted to the offspring. Learning to apply this process to the living human is quite big research topic. Obviously nature had to figure some kind of mechanism how to not transmit the cellural damage forward.

It is applied to the living human (or other being), when it is born.

Applying the process to already old people would be the abomination of desolation and turn this planet into a hellish dimension of unimaginable proportions, and it would of course exterminate humanity. You have to remember that when humanity is exterminated, that means forever.