The anime Ponyo Upon a Cliff comes out soon but a couple of stills from it kind of remind me of a few things.
The first is how in early years we play as a group together because the fun we can have together is the bigger thing so boys and girls play together easily.
Then as we start to reach five things start to change.The boy here is looking for an excuse to not play with the girls, it's quite probable he senses the social kudos he will get for waiting for a better off from boys as he's trying to make a stand for who he senses he ought to be with.
The girls can sense his excuse and call him out on it as any one who was five knows you have few chores beyond keeping your space tidy.
By the time they reach seven or eight the girls will be the same, only playing in tightly policed girl only groups so if if as was the case at times they boys were playing games that either I couldn't join in with or just didn't remotely interest me having a feminine side they would refuse to let you join in.
I know that cos it happened to me at the same time the girls parents were starting to push for equality for girls and there was a lot of noise about Boys dominating the playground.
Boys games tended to use more physical space but both groups could be pretty dominating and exclusory for boys and girls like me that didn't fit neatly in their ideas of what boys and girls are and should be.