Ok, but this doesn’t scale well. It takes a nontrivial amount of time to evaluate each person, and it would have to be done repeatedly as they age. You would need an entire government bureaucracy devoted to it. Then people will want an appeals process for when decisions don’t go their way—both parents and children.

99% of people are probably going to use the high school diploma route and not the demonstration route, so that's basically age with extra steps.