Why does my 12 year old never age?

Restrictions on a device have no relation to the Apple IDs age. They are set in Settings->General->Restrictions in iOS 11, or Settings->Screen Time->Content & Privacy Restrictions and can be changed there at any time regardless of age.

The only thing an Apple IDs age limits is the Ask To Buy feature of Family Sharing. It will be enabled by default on Apple IDs whos age is less than 13 years of age, but can be turned off if required:

See here for how to manage that: Manage Family Sharing and Apple ID for your child - Apple Support

As to the age of the Apple ID, where exactly are you seeing the age continue to be 12 years old?

Update the birth date associated with your Apple ID - Apple Support

The age on an Apple ID should continue to age as time goes by. ie. it is calculated based on Birth date, and is not a hard coded value.

there is a contact link.

Apple ID Support

Apple ID Support (2)

Apple Support by e-mail or chat Select your device, Battery, Power and Charging, then Power adapter not working to get a chat option. If the initial contact can’t help you, ask them to have someone who can help contact you.

Apple Support using Twitter

Or fill out and submit this form.
The form is for iTunes, but it will get passed to the ID Support section.