Shou5 na Kanojo to Otona no Ai

Overview

Carol (5th grade elementary school) likes Kotarou (25), the guy who lives in her house. Being aware of her own limitations she realizes that she, who's still a child, will not be able to win Kotarou's heart. Things get worse when she finds out that Kotarou is dating her friend's sister, Chizuru. She cries knowing that she cannot compare with Chizuru, who's tall, has plump breasts, and is an adult. That night, she writes many times in her diary "I want to be an adult", until what she wants is carried away in her dream. When she awakens there is a strange woman in front of her, claiming that she is Ai, the daughter of Carol and Kotarou who has come from the future.

Source: AniList