Asatte no Houkou.

There are far worse things than having to deal with your ex-boyfriend's little sister or your older brother's ex-girlfriend. But that's something Karada Iokawa and Shoko Nogami have to learn the hard way when a magic wishing stone inexplicably grants Karada's wish to be older - and does it by stealing the years from Shoko! Now Shoko, who's already graduated and studied overseas, is Karada's age, just about to enter junior high school. Meanwhile Karada must suddenly face the real facts about what being grown up entails. And just to make a bizarre situation even more awkward, there's the issue of how to handle their existing romantic interests!

(Source: RightStuf)