I wasn't talking specifically about Japanese series when I moved to serious stories, but re-reading my post, I understand why it came across like that. I was referring to overall stories in that case, my bad.
That being said, I do remember seeing/reading stories where that happens in anime/manga, and it is mostly because the kid's infatuation is not the main theme of the show. It is either a one-episode thingy, a literal recurring joke, or something so minor it's not even worth touching upon. A current example would be Spy x Family, where Anya's friend is crushing hard on Lloyd, but you know that is ultimately just a joke.
However, I can't disagree with what you say. Japan indeed has many stories of kids falling for another character many years older and still manages to get them, either after a "somewhat appropiate" timeskip, or, in the most sickly ones, after they "promise" to marry once the younger one gets of age (we have the infamous "Usagi Drop", or that teacher and student relationship in Cardcaptor Sakura, to name a few an examples). But, it is important to mention that, as far as I remember, in many of those cases, the relationship IS the main plot (like in the former example) and not a secondary or tertiary plot (like in the latter example).
But, there is also a few examples of the crush turning out to be just a phase, or at least, the young character figuring out it is an impossible love, and has to let it go, such as Hanamura Yochien.