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.)

“Twenty years,” Orochimaru says, cupping Sakumo's cheek, even though there's nothing soft in his expression. “Twenty years of looking for a way to bring you back, not just find you again. And all it took was a little boy with a handful of inspiring words.”