Nanaka 6/17 (2001)

Seventeen-year-old Nanaka Kirisato is a high school student serious about her studies and goals in life. She frequently criticizes her best friend, Nenji Nagihara, for being a childish delinquent who spends his time fighting other boys. Then one day, after a heated argument with Nenji, she falls off a flight of stairs and suffers a brain injury, resulting in her mind reverting to that of a six-year-old. With this in mind, Nanaka’s father and Nenji must keep her injury a secret as she struggles to live a normal life and grow up all over again.

Source: www.anisearch.com/manga/2017