Sync Merchant Center Error: https://support.simprosys.com/faq/how-to-synchronize-errors-from-google-merchant-center
Here is a quick link to install the app: https://apps.shopify.com/google-shopping-feed

On the diagnostics page in the Google Merchant Centre, you will find the list of products having multiple errors including “Missing Age Group”.

The error is highlighted in below-mentioned screenshot:

Here’s how you can solve this error:

• Click on “Filter products”. Select the option “Errors from Merchant Center” from the drop down menu.
• After synchronizing the errors, select ‘Missing Age Group Error” and Click on “Add Filter”.
• Apply respective additional filters to a particular group of products.
• Assign the right age group attribute to all the products having Missing Age Group error.
• If all the products are of the same age group, you can fix the error by selecting all the products at once.
• Once selected, choose the appropriate product age group from the drop down menu. 

• Click on “Apply in Bulk’.
• Now, click on the “Pending status” tab

• Open the product link in the Merchant center by clicking on the product’s merchant center link in the app.

In the Merchant center, it still displays “Missing Age Group” Error. 

• The merchant center will update the item status box within 3 minutes.
• Refresh the product page in the merchant center. You will see that the error is fixed.
• Now, go to Raw Feed Attribute of content API section, here you will see the age group attribute with the value assigned in the app.