The boy's 16 and the woman mid or late 20s?
Wouldn't exactly call this wholesome...
Likely around 22-24. It shows when he's 6 she was in middle school (12-14). I think a lot of these types of age gap stories only don't come off as too creepy when they're handled from the right perspective. She left her real home at like 13-14 after moving to Tokyo, ran away from her new home, never had a chance to be a kid anymore and then came back to find her grandma and her happiness gone, having never found it in Tokyo - except for Harumi. The times she was happy, their shared grandma, the nostalgia, and his overall maturity (outside of girls) really cemented things.
I don't care for age gap stories, but when they're done right like this one I can accept them. Shared history and time apart really is important to make them acceptable, because it doesn't become an exploitative relationship or groomed relationship.