Update 07/12/2021
As user3094434 states in the comment below, the information has now been moved to “General” > “App Information” > “General Information” from the menu on the left side.
Old Answer
In the tab App Store where you enter app information, you should be able to find it.
It is located under General App Information (containing both the App Store Icon and Version section) there should be an Age Rating you can click edit on.
When you click the edit button you will be prompted with the following window:
