Thirty year old Kakashi was supposed to have been killed by Pein during the Invasion. Instead, he wakes up in the body of his twenty year old self. (It gets a lot more complicated.) Time travel.

There's something to be said for keeping the past as it was - makes it predictable, malleable. Naruto never bought those arguments. He's always been the one to power through adversity rather than wait it out, so why would now be any different? He'll do what has to be done, even if he steps on a few butterflies along the way.