As the kid who stood 4-6 inches taller than the next tallest kid in my class until I stopped getting taller when I was 5'8" and 11 years old, I'm acutely aware that kids physically grow at entirely different rates even during school. (I'm a woman, so this is still above average height, but I'm nowhere close to as awkwardly tall as I was in a room full of 11yos.) Look at any classroom and you'll see a wide variety of physical development.
I'm just saying age is an arbitrary indicator for every category I can think of, and most situations would benefit from using a more relevant metric. In situations where size matters the most, let the huge 10yo play with the 11 or 12yos and the tiny one be with the 8 or 9yos. When I was 11, for safety reasons my karate instructor had me start training with the adult women who were closer to my size. In situations where intellectual ability is the relevant factor, you might have even a broader reach. If my parents hadn't refused, my kindergarten principal wanted to have me in 3rd grade by the end of my first year and doing two a year after that. Sitting through 13 full years of classroom instruction instead of 5 or 6 was miserable. Somebody you share interests with might happily overlook the fact that you're a couple years younger and still want to be friends. It's at least more likely than that someone will overlook the fact that you have absolutely nothing in common because, hey, you were born within a couple months of each other!
I'm just saying age is an arbitrary indicator for every category I can think of, and most situations would benefit from using a more relevant metric. In situations where size matters the most, let the huge 10yo play with the 11 or 12yos and the tiny one be with the 8 or 9yos. When I was 11, for safety reasons my karate instructor had me start training with the adult women who were closer to my size. In situations where intellectual ability is the relevant factor, you might have even a broader reach. If my parents hadn't refused, my kindergarten principal wanted to have me in 3rd grade by the end of my first year and doing two a year after that. Sitting through 13 full years of classroom instruction instead of 5 or 6 was miserable. Somebody you share interests with might happily overlook the fact that you're a couple years younger and still want to be friends. It's at least more likely than that someone will overlook the fact that you have absolutely nothing in common because, hey, you were born within a couple months of each other!