It's sweet, however...
The elephant in the room, the matter of the character's ages, constantly looms over the story like a dark cloud from the moment it's introduced, and never truly leaves. It's not great.
The cutoff age could've been 21 or 25, both are developmental milestones that would've both made sense and made the romance less... dubious, but it's 18, so it isn't.
If you're used to the weird anime-ism age thing, then this story is great, solid 8-9 out of 10. If you're like me and can't stomach it well, it's probably a low 7 at best. The story never really shakes the looming sense of unease the age difference brings in my opinion.