I just found that creating the new version and editing the ratings made the warnings at the top of the screen go away.
No Apple employee on here has offered the actual steps, specific information, or answers to any of our questions, so everyone here is just doing trial and error. If what I've done is wrong, then it's Apple's fault in two ways:
They didn't explain this process correctly at all;
The App Store Connect website removed the warnings when I made those new versions, so Apple's developers wrote it to do that.
"App Store Connect Engineer" said this (it's the accepted answer):
You'll be able to update your app's age ratings when you submit a new version of your app.
"New version", not "new binary".
Topic:
App Store Distribution & Marketing
SubTopic:
General