It's not uncommon for students to switch between top 5% and bottom 5% - yet people act as if its static. Education and chid development are ridiculously complex. It's not possible to have a solution that solves for everyone.

That's a good question that has several reasonable answers (also depending on context), so we should run some proper experiments to find which answers are best.

Though from a policy point of view, we don't need to know those answers: different schools should try different answers (including sending all kids to the same class), parents should send their kids to the schools they deem the best overall package.

As long as you permit or even enforce periodic rotation between the "tracks" when appropriate given the performance/speed of learning, you might as well start early.