In which denominations are Sunday school classes categorized distinctly by grade/age level? 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?
My experience has been that this is not so much a function of denomination as church size.

To wit:


I've preached at Baptist churches where there was one room for all kids - because there were fewer than 5 kids on a given Sunday.  (Remember, there are a lot of churches out there with less than 40 people!)
I've preached at an Episcopal church with 3 classes - one for under 5s, one for 1st - 3rd grade, and one for 4th - 6th grade.  Again, with a regular attendance of about 100, that was the best grouping that preserved both critical mass (you need more than one kid!) and some age separation (don't put 4th graders with 4 year olds.  Neither wants to be with the other.)  Indeed, when an influx of children warranted it, we rejiggered the age boundaries.
I've taught Sunday School in a Baptist church that insisted on one grade per age, and on those Sundays when your one kid didn't show up, well, it was rather demoralizing.
I've been part of a mega church that had three or four classes for each grade.  There, we used the highly biblical model of looking deep into the child's soul, and assigning them a class based on the first letter of their last name.


In short, this isn't a doctrinal thing, it's a practical thing.  You need enough kids to get an actual class, but you want to keep them about the same age too.  That's a function of church size, not theology.