13Her powers limit the rate at which she ages
The movies portray her as a child when she first encounters a young Charles Xavier. The two grow up together and Xavier sees her as something of a little sister. However, the comic-book origin of the character is vastly different.
In issue #17 of her solo series, she says: "I
wasn't born last night, or even last century, for that matter..." and has been seen as an adult at the dawn of the 20th century several times. This makes her one of the first mutants on Earth, much older than either Xavier or Magneto. In fact, there are only a handful that pre-date her, including Exodus, Selene, and Apocalypse.
Due to the nature of her shape-shifting cells, they are not inhibited by the Telomeres that are an essential part of how human cells age. Because of this, and the way her body constantly morphs and adapts, she can heal much faster than a regular human, too, albeit not as fast as Wolverine or Sabretooth. Therefore, she maintains the physical advantages of a woman in her prime, despite being well over a century old!