I've played Fortnite with my friends' kids before, but last night I played with my friend's 12yo son and _his_ friends and my mind is still spinning.
It's not that they're that good... they're not. It's that for them, the game is background noise. Fortnite isn't a game, it's a place.
They hang out in Fortnite the way we used to hang out in basements or back yards. We played games or kicked a ball around, but it was all a pretense for the social aspect.