Yes. Whatsapp requires a phone number and Discord does not. The tweens who do not have a phone yet can join Discord with their siblings / friends.

The other part of this is that Discord has official University hubs, so the college kids are all in there. You need an email address from that Univeristy to join: https://support.discord.com/hc/en-us/articles/4406046651927-...

Discord's initial core demographic was online gaming. From there it has radiated outwards due to being the best group messaging (and voice chat) solution out there. The more overlap your friend group has with gaming and adjacent groups the more likely they are to use Discord

Maybe, but at least in my circles it’s a structure thing- until the group actually can be organised in a single chat sanely something else will be used- but as soon as multiple chats are required the thing is moved on discord.

might be a regional thing instead, i don't know many americans with whatsapp -- all of my friends are on discord.