I've read a manwha some months ago about a boy in his late teens who just failed university entrance and suddenly find himself back in time in his 10 (?) years old body.
Does it ring a bell to anyone ?
Note : it's not "Again!!"
There's also a video which was on youtube, showing the end of a movie, sort of a "Big" lookalike, except in this one the woman decide also to regress to stay with the boy. It's was quite nice, although it lacked real process.