With an age under 13, that would indicate you could only have an account that is managed by an Organizer in a Family Sharing plan. It may be possible for them to change your birthdate, although it is also noted in this support article that it may require creating a new Apple Account.
Update the birth date associated with your Apple Account - Apple Support
If creating a new Apple Account is the only option, the Child account can be deleted with approval from the Organizer. Follow the steps carefully here and make sure you are able to sign out on your device, since after it is deleted, there will be no valid password to sign out. That leads to the nonexistent account being locked to your device requiring an Activation Lock removal request. All the details are available here: