(Ignoring everyone who talks about reversing genders and their age gap)
Since dating in high school is normalised in almost every anime, the boy is not doing any kind of sin.
The only thing that hits the sore spot for people is that the woman is 12 years older than him. They probably would have been fine if she was a year or two older but 12 is unacceptable for them. Can't blame them, it's just that they don't know how to appreciate a good age-gap romance.
For me, as long as dating is fine for the younger one's age, and the two maintain a very respectable relationship, I don't mind which one of the gender is older or how old they are. The beauty of age-gap romance is the relationship they create and how they treat each other. Not even standard romance stories could beat that level of relationship. That's why I don't watch high school romance animes (both in high school).
Let's just say the beauty of love is represented very beautifully in this movie. Something most love-genred animes fail to present, and stick with cliched romance.
