I'd say so, especially if you start on desktop and have them watch the 2-minute onboarding video. We are satisfied with what we see with our internal usability studies with nontechnical users.

Among customers, one reference that I can quickly cite is this one:

https://zulip.com/case-studies/gut-contact/

> Agents at GUT contact use Zulip every day to communicate with their team leads. “Most of our agents are in their 60s or 70s, so the software must be as simple as possible. That’s why we love Zulip,” says Erik Dittert, who’s been leading GUT contact’s IT team for the past 20 years.

I would recommend doing a little training/handholding call/video when moving over a community -- but this is true for any new app.

My mom needed training to do basic things in Squarespace, and I had a friend who worked at Slack whose manager started every chat message with "Hi <name>" and ended it with a signature, like you would an email. :)

Data point of one: in my small community group that has moved to Zulip we do have a grandma contributing. No jocks though so I can't speak to that.

I would also like to note that Slack did not pass the grandma test in our case. I highly doubt that Discord would given how hyperactive the UI is.