There are currently minor age partners. I presume that this has been handled for child actors for a century now so it honestly isn't that hard. The only thing I can think of is perhaps there's some COPPA (US law) issue that none of us are considering. That said, I doubt it and certainly a parent or guardian can enter a contract with Mixer.
It may not be practical to list all banned games outside of an easy target like AO rating, but certainly it might be practical to inform the streamer that a borderline case isn't allowed and they need to stop playing immediately. The streamer would be expected to stop and any protest to be taken privately offline instead of trying to embarrass the CAT agent on the spot.
Per the comment below... there is a warning in browser. I believe many Xbox users are still complaining (and perhaps mobile). Some folks take their follow length as a point of pride and it gets wiped. Admittedly channel bots could take up the slack, but hey we should be able to trust the Mixer API when it says something.
I would like to be able to click links in the chat when using Mixer app on Xbox (and have those open Edge). Why? Many reasons, but one is so a link to subscribe to a partnered streamer just works.
And when there is an Xbox Sub button? Well many times a chatter asks for information that can be supplied by links... maybe a blog post or something informative. A trusted moderator posts it, but then the chatter says "Oh I'm on Xbox". And the tears roll...