Asatte no Houkou.

This isn't me at all. Gokawa Kotomi, a sixth-grade elementary school student, lost her parents four years ago and currently lives with her brother, her only remaining relative. She believes that her existence causes her brother many troubles, so she wishes to grow up quickly. One day, Kotomi and Hiro happen to encounter Nozomi Shoko, Hiro's former girlfriend, in their hometown. Unaware of the relationship between the two, Kotomi kindly invites Shoko to go play at the beach. Suddenly transformed into an adult form, Kotomi, along with Shoko who has reverted to being a child, both feel extremely confused but decide to temporarily return to Shoko's apartment. However, Kotomi cannot accept this reality, while Shoko feels quite frustrated. On the other hand, because Kotomi did not come home all night, Hiro spends the night feeling anxious; seeing that Kotomi does not show up for school on the first day back, her classmate Amino Tetsuyuki also searches everywhere for her whereabouts.