A prevailing One Piece theory suggests that Jewelry Bonney isn't as old as she makes herself out to be. She's purportedly in her early twenties. However, that estimation is unreliable. The Supernova has a Devil Fruit power that allows her to alter the age of anyone and anything, including herself. This has led fans to believe the Big Eater's age is closer to that of a little girl. This is backed up by her words and actions throughout the story. However, nothing about Bonney's age is concrete.
Source：https://www.cbr.com/one-piece-theory-jewelry-bonney-child/
