It'd be even simpler. If a device is in Child Mode (which would be activated by parents during setup, and require a separate PIN to disable), it'd respond with status.isMinor = true. Or even simpler, make it a HTTP header.
What I meant is that it doesn't exist yet. It'd require operating systems, apps, browsers, etc, to all implement this system before a company like Discord can actually use it.
Eh, you're trying to boil the ocean. This functionality built into the browser would cover 99.9% of the use cases. Applications can be monitored separately, and I'm at a loss for why my OS needs to know about my age.
None of this matters anyway. If a 15yo boy wants to see boobs on the internet he's gonna find a way. There's so many ways to muck with the connection. Not to say these age verification checks work either; the recent usage of the Death Stranding character's face to bypass the checks is evidence of that.