I found out that the girlfriend I started dating is actually an elementary school student. Is this a problem?

Ishimi Shoji falls in love with Adachi Chisa, who attends the same calligraphy class, during the summer of his third year of high school. After a desperate confession, Shoji successfully starts dating Chisa, but... "...Chisa, why are you wearing a randoseru (school backpack)?" "Huh? Because I'm coming back from school." "...Sorry. But how old are you anyway, Chisa?" "I'm 11 years old." "Eleven?!" Though Chisa is very well-developed and mature-looking, she's actually an elementary school student. This is a story about a male high school student who starts dating a sixth grader, struggling against generation gaps (?) and social conventions.