If the content is mixed, it makes even more sense to have the content supply the age data. This is how it has worked with broadcast media pretty much forever. TV shows and movies gain their ratings based on the worst case on display. IE: a show doesn't have to consist entirely of swearing to gain a "language" warning, it just has to have some. Definitively mixed content.

I think your example exemplifies this. Among Us is not inherently adult-only, but since it's multiplayer, they don't control what other player say and do. Definitively mixed content. They should not be asking you to verify, they should be telling you and letting you decide if your kid can play.

I kinda can't beleive their lawyers decided to go that route and assume all the PII responsibility that comes with collecting that data, instead of just making the "it's online and there might be d-bags on our servers" rating much more obvious and explicit.