Asatte no Houkou.

Asatte no Houkou. follows the lives of Karada Iokawa, a young girl who is set to join junior high school after summer is over, and Shouko Nogami, a young woman who has just returned from studying abroad, who also happens to be the former girlfriend of Karada's older brother.

The day Shouko returns, she is dragged by Karada to the beach with her brother and a couple of their friends since she used to know Karada's brother Hiro several years before. After becoming irritated with Hiro for leaving her alone in the United States, she purposefully tells Karada that her ribbons are childish. This upset her greatly because she doesn't like to be treated as a child.

Later that same day, Karada is found by Shouko praying at a shrine, wishing to become older. Amazingly, Karada's wish becomes true and she instantly transforms into a young woman. Incidentally, Shouko then has her adulthood taken away from her and she reverts back to about eleven years old.

(Source: Wikipedia)