I do (and so do the Chinese; it's the way their system works, along with the teacher following the cohort, which would also be good). I think, realistically, most students will be within a year or so of their age cohort in most classes. If somebody falls more than two years behind, they probably need to be on a different track, anyway (e.g. they likely have a learning disability).

Teaching becomes easier, because teachers aren't trying to intermingle challenging content for more advanced learners with simplified content for less advanced. Instead, they can focus much more tightly on the student's level. Since teachers aren't dealing with bored "advanced" kids and bored "delayed" kids, the impact of class size is reduced.

It adds a burden to the administrators, since they would be responsible for determining the cohorts, but, after watching my wife be heavily involved in school finances, I strongly believe there are more than enough administrative dollars floating around to deal with it.

Finally, it's not the end-all solution. But I think it's a better framework than the current one. Even in the "gifted" program at my kids' school, you can see (and the teachers do an amazing job of accounting for) that some of these "gifted" kids might be amazing in math, yet still struggle at grade-level or a year behind in English. In the "mainline" classes, the inverse might be true, but those kids don't get the extra benefits of the gifted program since they couldn't test well enough on the mathematics.