Juku Juku no Mi in One Piece

As the fruit hasn't yet been shown in the anime, we don't know a lot about its appearance.

This fruit allows the user to touch an object and make it age and decay. When the user touches the ground it creates a sinkhole and it allows them to sink their enemies into the ground. This fruit can also help the user get past almost anything by aging them and causing them to collapse or be easily broken. This power can also be applied on living beings which can make them grow older in an instant.

The decaying rate will continue to progress once it starts and it doesn't have an end. If the user does this to a living being, then the affected person will age physically but not mentally. They will retain their original mental maturity. This process is not reversible at all. Other than that they have the standard devil fruit weakness, that is the inability to swim.

Shinobu uses her devil fruit powers for combat. She refers to her devil fruit power as jutsu. When she uses her devil fruit powers on living beings to make them older, she usually doesn't like using this as even though the person has matured physically, they haven't matured mentally and also there is no way the aging can be reversible. This can be seen when Momonosuke asked her to help him age and she refused.

The techniques used by Shinobu are:

Sexy Ninpo: Kare-San-Sui

In this technique, Shinobu touches the ceilings in which she hides and makes them collapse in order to escape.

Q. Why doesn't Shinobu like to forcefully age humans?

A. Shinobu usually doesn't like using this as even though the person matures physically, they don’t mature mentally and also there is no way the aging can be reversed.

Q. Who ate the Juku Juku no Mi?

A. Shinobu ate the Juku Juku no Mi.