Tsunade of the Legendary Sannin wasn’t one to age gracefully.
But she had no desire to look as old as she truly was so she used her unparalleled medical skills to mask her aging appearance. It’s an advanced technique which allows her to transform her body to adopt a more youthful appearance. Whenever she desires, Tsunade can change herself to look like a child, teenager or a grown woman of any age as long as it’s younger than her actual age. The change affects her both internally and externally to alter her height, weight, bone structure and body shape to match what she looked like at different stages of her life.
However it is limited in that she can only ever look like a younger version of herself rather than another person entirely. And more importantly, the change only affects her appearance without actually making her any younger. For instance, she would live no longer by shapeshifting into herself as a child for instance and it isn’t able to erase any injuries, scars or illness she might have. But her specialization in medical ninjutsu allowed her in good physical condition despite her chosen lifestyle after retiring as a ninja.
As further testament to Tsunade’s skill, the ninjutsu is maintained automatically by siphoning a small fraction of her chakra reserves rather than her consciously using it, meaning that the effects don’t disappear when she’s distracted or unconscious. It only when she experiences chakra exhaustion or uses Creation Rebirth extensively that she reverts back to her natural appearance. And even then the ninjutsu restores itself once she recovers.
Tsunade became more and more reliant on this ninjutsu as she got older, particularly as the after-effects of practising her Creation Rebirth meant that she was aging faster than normal. She even developed a habit of making her default form look younger the older she got.
While this technique has no official name, it would most accurately be called the Rejuvenation Jutsu.
The Narutoversity 