There is no age based criterion but it would be possible with a custom user field, which the member themselves would have to check once they reach 18.
Again, that can be done with custom user fields.
A wrote a guide (which you won't have access to until you're a customer).
[URL unfurl="true"]https://xenforo.com/community/resources/using-custom-user-fields-with-user-group-promotions.4008/[/URL]
I believe so (I've never done a gallery conversion).
If it is set as a Container type, yes.
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That would require some form of custom development.
The only ignore function built in is for members.
