In some areas of Manhattan, before birth.
Out in the heart of the Sandhills of Nebraska, ours was a gradual integration, everyone easily absorbed with no problems at all.
I grew up at the tail-end of the heyday of the one- and two-room country schools, of which the county had scores at the time. Up through the fourth grade, I went to school with only other town kids.
The one- and two-room country schools, dependent upon the "staff," ended with the fourth grade, the sixth grade, the eighth grade, or the tenth grade, after which those kids were bused to school in town.
When I started out, there were maybe forty of us kids in the town school, in two classes for each grade. By the time I graduated from high school, there were over a hundred (and oddly, 69 boys and 32 girls--an imbalance that didn't hold for classes before or after us), all the rest having come in over the years from the country schools.
Trust me, there was just as much a cultural, socio-economic, gap between town kids and country kids, as there was between inner-city blacks and working-class Irish of Boston.
But because the transition took place slowly, and wasn't involuntary, by the time we were juniors, seniors, in high school, everybody was best buds with everybody else.