From the support article: https://support.apple.com/HT204164
To change a birth date to any age below 13 (age varies by country), you must be a member of a Family Sharing group [ https://support.apple.com/kb/HT201088 ], and your family organizer must approve the change. After you change a birth date to any age below 13, you can't alter it except by contacting Apple Support --> https://support.apple.com/contact
1. Follow the instructions in the article to update the birth date on an Apple mobile device or on the web.
2. Follow the onscreen steps to send an email request to your family organizer.
3. Your family organizer will get an email with instructions to approve the request.
Read the document for complete details.