A majority appears to be:

62.7% of the Fortnite players are from the age group of 18 to 24.

https://www.demandsage.com/fortnite-statistics/

It's not just a kids game, has been around for long enough (seven years) and has enough staying power that even many of those who started playing it as kids are now adults. Personally, I never cared about it, but that's because I stopped playing multiplayer shooters a long time ago, not because I feel like I'm too old for this.