The upcoming "X-Men: Days of Future Past" allows the two casts - the young "X-Men First Class" and the older original trilogy group - to come together thanks to a bit of clever time travel.
In the original comic mini-series, the character of Kitty Pryde is the one to go back in time to stop a post-apocalyptic future from happening. In the film though, it's Hugh Jackman's Wolverine whose consciousness is transferred back into his 1970s-era body with the help of Kitty's powers.
One of the film's writers, Simon Kinberg, explained to Total Film why that decision was made:
"We made the decision for a lot of reasons, some of them obvious and some of them more nuanced, to make it Wolverine who goes back in time. One reason is that he’s the protagonist of the franchise, and probably the most beloved character to a mass audience.
Probably 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 that’s sent back, but because we cast Ellen Page in X-Men: The Last Stand, you’re talking about an actress who, in the age of Michael Fassbender and James McAvoy, would have been negative 20 years old.
So we started thinking again. The first reflex response to that was a character who doesn’t age. Wolverine is the only character who would looks the same in 1973 as he does in the future."