Athletics is one of the most global sports of all. Running in particular is accessible to most people so makes even more sense to judge across the same global standards which is age as opposed to school year when this changes over at different points in the calendar in different countries and continents.
Same principle as the mile being run in no major championships and only a handful of notable meets per year yet discussion about High School mile times seems to dominate the thoughts of many posters. Maybe US runners would punch their weight globally at middle distance if they consistently raced the 1500 at their development stages.