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 no such thing as a butt that's too big," Naruto yawned at her, wiping the drool from his mouth as he shifted over to give Sakura a little more room to fit. "All butts are beautiful because I say so. I'll make it a Hokage Decree. I, Uzumaki Naruto, Hokage of Konoha, proclaim the beautiful bounty of bodacious butts a blessing unto this village." (Sequel to Blind Stars.)

Kakashi tilts his head, gray eye analyzing her carefully for tells. He finds nothing. "I've never seen you before in my life," he says flatly. (Kakashi wakes up fourteen years old.) kid!fic/de-aging