@Legofrans You could search about Aristotle’s Philosophy of Friendship, same could apply to love, after all love is just friendship with more deep.
Aristotle outlined three kind of friendship
-Based on utility, seeking benefit from each other
-Based on pleasure (you could add sexual and romantic here if you not talking about friendship)
-And based on virtue, appreciate their trait and quality (This one last much longer and still reward with benefit and pleasure)
Why I ship this two because their trait really suit each other, not because Koiwai see Fuuka is cute or attractive but she is dilligent , caring, friendly, funny and yes, still very cute.
That's why age is not border of love. But you still consider two factor above, love is advanced friendship, so it required all. Still applies, Koiwai is handsome, rich ( freelancer translator have pretty good income), Fuuka is cute, motherlike to Yotsuba and pretty attractive, well, she thicc)