Schools group together only one age of kid for socialization and only 20-30 of them. If your kid is not into the same thing as enough of the other kids in that group, they will likely be ostracized, even unintentionally. So you must let your kid do the things their friends do.

Broader society does not restrict the age of who you can socialize with. My friends vary in age quite a bit. My friend's kid can play with my kid despite being a different age, but that's much less than the 30 hours a week spent in school.