Here are the questions that the developer answers when they submit the app that determines the age rating. For example, when submitting the app if there is no objectionable content, the developer simply checks that box and the rating will be 4+.
Age ratings - Reference - App Store Connect - Help - Apple Developer
The only way Apple would require a developer to change their age rating would be if a user reported content that was not disclosed and in that case the developer would be instructed to either remove the objectionable content or update the age rating. The content also includes the ads that the developer chooses to run as you would not want a children's game to display ads that have objectionable content. Again, it is only the developer of the app that has any control over the content displayed to the user and how they answer the questions to determine the age rating.