… the bigger reason is that when we started thinking about the logistical realities of Kitty’s consciousness being sent back in time, to her younger self, as opposed to her physical body being sent back..it was impossible. Obviously in the book it’s Kitty..but you’re talking about an actress (Ellen Page) who, in the age of Michael Fassbender and James McAvoy, would have been negative 20 years old. So we started thinking again, and the first reflex response to that was a character who doesn’t age. Wolverine is the only character who would look the same in 1973 as he does in the future.
”—Simon Kinberg (Writer, X-Men: Days of Future Past)