Our new pair of toe-rags are now aged 16 and 17 weeks - we got them at 12 and 13 weeks (not littermates) They're both boys.
Ever since we've had them, they've treated their litter trays as arenas for wrestling and playing - they'll just jump in and start a playfight. They also play chase all around the house and will do laps of the house, running right through their trays as they lap the utility room.
There's spilled litter everywhere, plus it's not very hygienic to have them traipsing bits of litter and poo all over the house in their fur - and they're sometimes a bit whiffy from rolling in their mess.
They do also use the boxes for toiletting and don't seem to have any trouble distinguishing them as the places to go. No accidents. I've never known cats to be so unfussy about playing in their own poo. We bought a covered litter box hoping this would stop the behaviour but they were reluctant to use it and clearly not very comfortable, so we put the open trays back down.
We can't let them outside until they've been neutered in a couple of months - any tips for stopping toilet-play in the meantime?!