Cannot select target age group inside Play Console

Question

After submitting my app to alpha test track I received a an email with message: "Although you haven’t included children under 13 in your target age groups, we determined that some elements of your store listing may appeal to children under 13. For example, your store listing contains elements such as: Animated characters in the app icon. Young characters."

I wanted to change this but selecting age groups below 13 years is disabled.

How can I add age groups below 13 years for my app?