>So at what age should we constrain a student's destiny?<
Second trimester.
Imparting competencies and assessing them for record should be completely divorced. You should be able to go to the post office and take an examination to obtain certification that you are at level n of subject matter X. Yale students should have to go to Harvard to sit for their quals. Students that are making the prescribed progress per postal exams should not be required to attend school at all.