Do you have any basis for assuming that that users who are under 18 will have that information revealed to other users? That’s not what I read in this article.
I also think you’re being quite prescriptive about a private commercial service. You’re entitled to your opinion, of course, but companies enact restrictions to their products and services not required by law all the time.
If Discord wants to make their service require a mandatory declaration of allegiance to the Klingon empire, they’re allowed to do that.