Wherever the story goes, it will be painful. Whether they end or not together.
Somehow I'm not too cynical about the age gap concept given. I'm not a pedophile, alright.
Basically the idea is the same wherever someone's age is, there will be a distance that separates them. Between 17 yo and 24 yo, 20 yo and 39 yo, etc. The problem is, Mashuu is still underage.
On the one hand I don't see this as your typical Shoujo Manga that all goes full with fluffy and sweet scenes. And from this manga you make Mashuu x Satoko as your OTP, no.. this is completely different. Everything is taken in a considerate perspective, a more realistic approach with anxiety between love and the needs of one another. This is a concept that was given from the start by the author. Not seen in terms of crime, pedophiles and the like. I don't see this as something disgusting just because you love someone older. This is not hentai, all considerations are different from the relationship of sexuality
Satoko is a person who is full of doubts, skeptical, she is portrayed as someone who carries all the burden on her shoulder. Most likely she will reject Mashuu. But everything will be different if the story goes to a more serious route.
Satoko needs someone by her side, embrace her, calm her. These are points that will be dragged aside from age problems. The safest route is, everything takes time, the development of their relationship will be seen when Mashuu grows up. Mashuu will try to grow into an established, independent, responsible man...and this is a form of sacrifice I want to see.
Let's see from the needs of the condition itself.
I honestly like the title of this manga, My Boy
This can come from various meanings. Someone gets encouragement, inspired. Mashuu is the person who comes into her life, there is a possibility that future developments will change the meaning and outlook of her present life. "My Boy" is a picture of pride. Mashuu is the key of the whole story.