I disagree with OP that a lack of kids is the reason why Lemmy is good. You're right that the level of maturity is not attached to age, and that we can find a platform that's fine with either age group.
But I do think kids absolutely do have the capacity to ruin things. Discord is basically a worthless platform depending on what fandom they focus on. You can usually tell you're in a shittified community ondiscord if
There's a vent channel
There are twitch emoji in active use
These may sound like generalizations, but, as soon as I got them pointed out by others (typically by discord power users who've seen the worst of it) I also started noticing the kinds of typical users who those things serve, and they're precisely the least mature and most childish company you could ask for.