Accelerated tracks would produce the top tier, which begin in elementary school - so it's a matter of how much your parents invested in your education before school. Any child can technically enter the accelerated track at any grade. The later they join, the more untaught expectations there are. The other students went over these things already in previous accelerated classes. There's no on-ramp.

In the normal track, you don't eventually take calculus in math, learn much about labwork in science, or even learn how to write a research paper until the last year of classes at 18. (Source: class of 2005, USA)