Quote
age classes totally removed
Agreed! If schools focused on readiness/ability for cluster grouping students in each subject, it may be a more natural learning environment... more similar to college, workplace, community, family, in which those one interacts with on a regular basis are of different ages and diverse interests. Clustering by readiness and ability in each subject may enhance a teacher's ability to present material, support learning, and address topical questions in a meaningful way. Clustering by readiness and ability may help provide intellectual peers for gifted learners. A win-win scenario for students and teachers.

Quote
In our area, I see many more boys than girls being held back. While "maturity" is usually cited, it really seems like "larger for sports" is a big concern, with a thought that academics will be better coming second.
I see this as well. Despite potential causal relationship between sports related concussion and brain damage, some in society want certain team sports. For others this may be reminiscent of brutal "entertainment" throughout history, providing a fight to the death: coliseum, gladiators, knights.

While there may always be red-shirting, and it may be an appropriate choice for some children, the wide-spread endorsement of the practice (not the practice itself) may come into question: To the degree that red-shirting is so prevalent that it may influence decisions not to provide a grade-skip to other children, red-shirting may be detrimental. When school policy/practice does not support the academic/intellectual growth of a student to progress at their demonstrated comfortable learning pace but focuses on demographic characteristics such as age (which at best is a proxy for statistical "average" of development), students have lost their individualism and are being educated without regard to their personhood; It may be wise to question the direction and motivation of the educational system.

Red-shirting does not create a "gifted" child.

Rather than getting bogged down by focusing on paths others have taken, parents of gifted children may wish to band together with a voice that seeks appropriate academic/intellectual curriculum and pacing for the gifted.

Because many participants on forums may not be familiar with other systems, it may be difficult to consider a viewpoint other than the educational system with which one is familiar. Here is one small tidbit: Singapore is rated as one of the freest economies, and one of the least corrupt governments of the world. Some may see its growth, in part, as fruit of its educational system. An educational system which nurtures gifted. While not holding up Singapore as a panacea, its Gifted Education Programme (GEP) may offer some ideas worth adopting.