Legality isn't even an issue as Natsuki has turned 20 already. The age gap might be.. But after you hit 20 that kinda age gap doesn't even feel that much.. (Me speaking as a 25 year old who feels like younger people are getting more successful. I can relate more with 30 year olds ╥﹏╥)
I didn't expect it to be so good. In the end it gave me a feeling of nostalgia.. High school life is indeed full of hopes and dreams. (▰˘◡˘▰)