See What is Sign in with Apple? - Apple Support
"Sign in with Apple is not available for children under 13 years old. This age may vary by country or region. This age restriction doesn't apply to Managed Apple IDs."
If you still think it should be preventing you then refer to this document for ways to contact Apple --> Choose your country or region - Official Apple Support
Select your country (also look for "other" regions), then a product. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.
or:
Contact Apple for support and service by telephone --> Contact Apple for support and service - Apple Support