A new policy requires you to set the age target, or the Store may remove your app from the store. In my case it simply refused to update it.
Searching the error returns this explanation at https://developer.android.com/google-play/guides/families:
All developers need to declare a target audience for your apps in the Target Audience and Content section of the Google Play Console. If you indicate that age groups under 13 are not in your target audience, no further action is required in the Target Audience and Content section.
Upon arriving to the Play Store Publish tool, I get this explanation:
The questionnaire asks who I am targeting. For people under 18, you have to comply with a few laws. Since I didn't have time to read through the laws, I went for 18+ for now.