How to fix Family Sharing not working due to age verification issue on iPhone?

Did you use one of these options to verify your age?

...

When creating a new Apple Account or using Apple services, you might see a prompt to confirm that you're an adult. This is required by law in some states, countries and regions.

In certain states, countries, and regions, you might be required to confirm that you're an adult before downloading apps, changing certain settings, or taking other actions with your Apple Account.

Learn more about age requirements for managing an Apple Account by country or region

Depending on your state, country, or region, you might be asked to confirm that you're an adult when you create a new Apple Account, update the software on your device, try to change certain safety settings, or try to download or purchase apps with an age rating of 18+.

...

Different options might be available to confirm that you're an adult:

• Existing Apple Account information — Apple might ask to look at information associated with your account, including whether you have a credit card on file or how long you've had an Apple Account to help confirm that you're an adult.
• Use a credit card — You can add a credit card that belongs to you to your Apple Account to confirm that you're an adult. If you use a credit card, you have the option to add it as a payment method for your Apple Account. If you don't add it as a payment method, your credit card isn’t stored. Debit cards aren't eligible.
• Scan your ID — You can scan your passport (available in Australia, Singapore, the UK, and Texas in the United States)*, driver's license or government-issued ID to confirm that you're an adult. To help scan your ID successfully, make sure that you're in a well-lit place (indoors works best). Place your ID against a neutral background and avoid glare. Make sure that the image of your ID is clear and in focus before submitting. In some cases, your ID might need additional review. If you choose to submit for this review, you'll receive a notification when the review is complete. Apple and its service providers use the image of your ID to confirm you're an adult and prevent fraud. The image of your ID is kept for a short period of time to complete this process, after which it's deleted.

*Regardless of your state, country, or region, Digital ID in Apple Wallet created using a U.S. passport can also be used to confirm that you're an adult.

...

copied from: If you're asked to confirm that you’re an adult - Apple Support

Did you also try to log into the Apple Account of your child and change the age listed in the personal information?

Update the birthday associated with an Apple Account - Apple Support

Also have a look at this recent discussion:

How to update a child's birthdate in Fami… - Apple Community

Yes and has his age but I doesn’t let him join my family sharing,it says I need to confirm my age but I alr tried multiple things none work and also idk if it’s because he put adult 18+ when making his account but I saw that you have to join family sharing so it can switch to supervised acc