Can a minor attend a Genius Bar appointment alone? Can someone under 18 attend a Genius Bar appointment alone, if the device is in their name and they know the Apple ID details?
No, you need to be over 18. They might accept a minor with ID but it is not guaranteed:


  1.7 No Sale to Minors. Purchase under these terms is available only to those who have reached the age of majority.


(Source)

Also confirmed by a recent live chat.