This has some dark "American beauty" kind of vibe. Not sure do I like it or not. But its definitely is interesting on it's own way...
P.s. the most thing is worrying me not even huge age gap, but that she is only 15 (he mention that her graduation takes 3 years which means that she in her first year of high school only) - I would me more... calm? If she were about to graduate - but this feels a bit horrible even for manga standards (I mean a red a lot of stuff like "older men and high school girl", but all of it was pure and romantic, not so mature, and Idk how I'm feeling about it...)