1

Can someone under 18 attend a Genius Bar appointment alone, if the device is in their name and they know the Apple ID details?

2

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.

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