Although the exact origins of Mystique have never been explored, it has been established that she was active at the start of the 19th Century, and a dream sequence once showed her on a mission in the 1700s. Her long-term relationship with friend and lover Destiny supposedly started at the beginning of the 20th Century, and doesn't appear to have aged since. It's also been established that she knew, and had a relationship with, Wolverine several decades before he went through the Weapon X program. While this by no means makes her the oldest X-men villain (Apocalypse and Selene were both operational in ancient times), it does leave the potential for Mystique to be much older than anyone realises.