In our area, competitive team sports are strictly by age. You can "play up" (young for grade) if you are very good, but cannot play old-for-grade, as there are closely-enforced birthdate cutoffs (usually in two year brackets). So redshirting only affects sports before third grade and after eighth grade. That's both too early and too late for me to use sports as a primary determining factor for grade placement. Of course, we homeschool, so, in practice, we report grade levels according to the activity.
