Some schools practice the concept of "mixed age groups" where each class is composed of 3 different ages in equal proportion. For instance my 5-yo son has 3-yo's and 4-yo's in his class.

Thing is, it's a constant battle for the school with parents who always want their kids to with same-age kids or older-ones, never smaller ones. Everyone thinks their kid's development will be "held back" because of the younger children in the class. Telling them about how older kids also learn from playing or working with younger kids is a hard sell, when all parents think today is academic progress.