Everything has already been written. I think it's purely a question of personal taste. Here are the two things that didn't work for me :

- Age - gap : I was very worried about that aspect. To be fair, it was handled rather professionally and there was no reciprocity until the FL was an adult. But it still is a bit on the edge for me. 19-24 is not huge in theory and a 5 years gap is pretty common later on in life. But people change A LOT during those specific years. I am 24 and would personally never consider looking at a 19 yo coming out of highschool as anything else but a young adult who has a lot to learn, no matter how "mature" they present. It would just feel wrong to me.
The ML has treated the FL like a little sister for 10 years and even though they weren't always together, they still were close enough to have each other's phone number and occasionally text. The shift in the relationship almost as soon as she becomes an adult is a bit abrupt for me.

- the ML approach : once the ML quite suddenly decides that he's gonna go for a 19 yo, he gets very blunt with his approach. And it makes sense, he's honest and clear about his intentions and it's probably the best way to straighflowardly break the existing walls between them. I personally don't like this style at all, it's a matter of personal preference but he came off as too pushy and rushed for me.

Other than those two major contention points in the story, it's a very well made, peaceful love
story. So if you don't mind the age gap and the straightforwardness, it's worth a watch.