There are several glitches in the game that does reset the number of days within the current life stage: Life fruit drinks, makeovers, plastic surgery, edit in CAS(without changing lifestage) and tattoos. But they are glitches in the way that the game isn't supposed to act that way. Technically, as far as the game is concerned the only way to truly make someone younger outside of a cheat is the Young Again Potion, eating a life fruit for one more day or ambrosia which resets to the beginning of the lifestage.
Yes, you can edit a Sim in CAS to change their life stage but doing so can also remove Lifetime Happiness Rewards and hidden traits.
