The age gap itself isn't the reason I have issues with this. The issues arise because of power dynamics. Realistically, she has more power than him and he is incapable of consenting to a situation he doesn't fully understand. Without a clear break longer than the two-year gap we got, this dynamic can't be altered. Even then, I have my reservations. If they met at 24 and 42, I'd be 100% ok with it.
Manga like this is useful for exploring the reasons behind your morality. If I can't describe why I believe something is wrong, I tend to believe that it isn't wrong. In this case, I can explain why I think it is wrong, so I'm satisfied.