There was one time, a long time ago, somewhere between mid- to late-1990s, I went to a church with family at night. I thought the adults would worship on the first floor, while the children were taken to the basement, where there would be a cafeteria that led to both the kitchen and the classroom. There was only one classroom. No room for another. All the children went to that classroom. There was a chalkboard, books, posters, a big desk for the teacher, and some smaller desks. For a while, that greatly influenced my perception of church layouts. The narthex-like hallway that led to the sanctuary or nave on the first floor, everything else in the basement.
Fast-forward many years to the early second decade of the 21st century, I visited a Baptist church, a Methodist church, and a Lutheran church, and they all seemed to have many classrooms, arranged by grade levels. Only the tiny family-oriented non-denominational church had a kids room, which was called a "nursery", empty and outgrown. And for the Catholic church, I couldn't find a classroom anywhere except the detached but affiliated Catholic parochial school nearby, presumably filled with many classrooms. In the basement, the Catholic church had a gift store. There were children but they participated in worship with adults.
Anyway, since when did churches begin to categorize religious education by grade level? And in which denominations is this so?