The idea of “age appropriate” in education is really quite strange - provided you accept that each child will develop in a unique path and at their own pace (and trying to make them go on a different path and/or at a different pace is inherently coercive)
What is appropriate for a given child depends on the child and their circumstances. Only the child can decide whether the educational resources provided are appropriate or not for their needs at any given time. This doesn’t mean they have to make this decision alone - they can be supported in that as well, if they wish to have that
However, this can’t be done in an education system that is really no different to force feeding geese for foie gras, where children are organized by age for the convenience of those doing the force feeding. (And within that, differentiated instruction is a farce - just stop force grouping children and allow them to organize their groupings according to their needs)