We split our boy/girl twins in PK at age 4.
They ended up in different schools in 7th grade and on (single gender schools).

3rd grade- I have fraternal twin boys who were in the same class previously (we lived abroad, they went to a small American school with only 1 class / grade). It was a tough transition when they started in separate classes in FCPS but I have already seen tremendous growth in aspects that they used to compensate each other for. However, it seems to be up to the parents in our district as there is another set of twins in the same class as well

There are two sets of fraternal girl twins in my DD’s small parochial. One family has them separated and one has them together. They are in 8th grade now. The together family’s mom has said she does it to make it easier on herself. I think that’s lazy and a real disservice to the kids.