I'm pretty sure by age 12, you can pretty much tell who the academic stars will be, who is mediocre and who the lazy slobs are.
This is a dangerous attitude, and I think it's actually one of the strengths of American education that we don't adopt that attitude. I think the ideal high school experience is a combined, diverse high school for everyone--but with lots of elective hours and extracurriculars so that gifted students can advance faster and follow their interests.
The notion that "by age 12, you can pretty much tell who the academic stars will be" leads to bad systems like the one that's common in German-speaking countries. You have a Gymnasium for your "academic stars", who are generally college-bound. You have Hochschule for the rest, and you have Lehre (apprenticeships) that culminate in eg. a plumber certification, or an electrician's license.
Students are split like that in what we'd call the 8th grade. This is thankfully a bit older than "age 12", but it is still far too young. Seeing some of my 16-year-old relatives who are already set for a lifetime career as a welder makes me sad. K-12 education with "forks in the road" hampers freedom and it represents a waste of potential.