Here’s the thing about Bonney. She is extremely gluttunous, has a resemblance to Big Mom, and her devil fruit manipulates age.
Now from what we’ve seen of Bonney, she can alter her age at will being able to change from a fully grown woman to a child or an old lady. Now, the thing about this ability is that it makes it hard to tell what her true age actually is and essentially gives a pseudo-immortality.
I say pseudo-immortality because she can manipulate her age to always be in her prime as opposed to other characters who will continuously age as time goes by. Time has no hold on Bonney.
Not only can Bonney manipulate her age, but she can manipulate other people’s age as well as seen at Sabaody where she turned several Marines into either little kids or senior citizens.
Age affects all beings no matter how strong you are and eventually you start to wear down. Case in point, Whitebeard. I think we can all agree that Whitebeard was at his weakest during the time of Marineford. Some people have even gone on to say he was the weakest Yonko during that time. We also know that Whitebeard was a beast in his younger days being able to go head for head with Gol D Roger.
Bonney’s devil fruit could have brought Whitebeard back to his prime perhaps during the time of Marineford. Just imagine how much damage Whitebeard could have wrecked if he was younger. Yeah. Terrifying.
Bonney could probably turn the Yonkos into kids or make them senile. I don’t think she’d be able to defeat the Yonkos with her devil fruit alone, but she could definitely wreak havoc. I mean imagine this. Bonney storms onto Kaido’s base of operations, turns his commanders into children so young they can’t do anything and turns Kaido into a baby. She then throws baby Kaido into the sea to drown.
*Theory by Bionic-TitanX