After a peek at your website, I see I'm quite a bit younger than you. I know a lot of people who do not have much money, but they still consider it worth it to spend a chunk of their limited income on Discord Nitro. In some servers I've even seen young teens (13-14) spending their allowance money on it. It would be interesting, based on your anecdote, if most Nitro subscribers are young users. It might be due to the custom emotes, which are probably more desirable to younger users than older ones.
On top of that, Discord has become the main social media platform for some people. I can't imagine using it that way myself, but I know a lot of people who are in a massive number of servers and spend much of their day on it. It's not just gaming voice chat for them, it's an entire social network. These people, who also tend to be quite young, almost always have Nitro, because it's worth paying $5-10/month for their main online platform.
Maybe the main area of concern for Discord should be whether these young users keep their subscriptions as they age.