My oldest nephew will turn 18 this year. When the invitations go out, he will be a senior in high school and still living at home. Will the rule still apply that he get his own invitation? Also, assuming he and his girlfriend are still together (at this point it's been over a year), will she get her own invitation at her parents' address (she lives with them and will continue to while in college) or can I just add her name to nephew's invitation? OR does the fact that he will have a plus one indicate he should have his own, regardless of living at home?
This is one of those instances where in my area it wouldn't at all be side-eyed to have one invitation for everyone at the home, regardless of age.
And one more.....
Keeping in mind the Indigo Child situation

, at what age is it appropriate for someone to be allowed a plus one? Example, my other nephew is 13 and has had a girlfriend or two. If he has a girlfriend at that time, should he get a plus one?