What age group often tends to split into distinctive subcultures with pejorative names for one another?
Early teens. Kids tend to move into groups of like interests and personalities and that's when the nicknames start to come out. I'm 63 and the group of guys I bum around with still call each other by the names we came up with at that age.
starts around the early teens (13-16) and goes until 20s. sometimes even longer.