I've heard that the people in the US who have developed a math phobia or who think that they have an inability to understand math self-determine this fact after particularly tough math concepts are being taught. IIRC, learning fractions is one of the first of those, and creates the largest cohort.

I believe it. I've known a lot of adults who don't quite get fractions. They generally also have no trouble working with money and giving change, but no ability to understand interest, especially compound interest. Somehow, the educational system is failing here, not the kid.