Sheryl is NOT 66 years old!!! She mode a contract with owl when she was of age to!!!
No offense to whomever had answered before me, but Sheryl's exact age is unknown for now. However she an Rufus are nearly the same age, and Rufus is 67 himself, making it possible that Sheryl is 66 or slightly younger because Rufus is the eldest Duke. Sheryl also didn't form her Legal Contract with Owl until quite recently, hence her older appearance. A Contract stops the physical aging process, meaning that Sheryl likely formed her Contract with Owl whilst Oz was still in the Abyss. As well, not having a Contract with the Dukedom's Black Winged Chain doesn't mean a person isn't the Duke. Oscar didn't have a Contract with Gryphon because he was unfit to form one, and Bernard didn't have a Contract with Raven because Raven could only be Contracted to by Gilbert - yet both of them were the Dukes of their respective Dukedoms. Sheryl would've been the Duchess from the moment the last Duchess (her mother or grandmother most likely) died. She would've had the Key and would've spent time developing the skills necessary to form her Legal Contract with Owl.