Devices don't have ages. They age is set on the Apple ID of the device user through the birth date.
You can contact Apple support by clicking the following link to have it changed to an age over 13 if required.
click here ➡️ contact Apple Support to thew change the birth date of a child under 13 - Apple Support
click here for more information ➡️ Update the birth date associated with your Apple ID - Apple Support
The Birth Date / age of the Apple ID, has no bearing on the restrictions set through family Sharing on the device. You can set any restrictions or remove them regardless of the age set for the Apple ID.
The age only applies to the "Ask To Buy" setting.