Inviting the significant others of guests is a basic etiquette standard. But at what age does it become mandatory? I would think inviting the "girlfriend" of a nine-year-old is excessive, but what about a 17-year-old? 15? 12? Is it anyone adult enough (18+ and/or living independently) to warrant their own invitation vs being invited as part of "The Smith Family"?
This is purely a theoretical question for me as there will only be one person--the youngest of my cousins--invited to our wedding who will be under the age of 18. She'll be 16 and if she has a SO, they will get an invite as it is literally only adding one person. (I'd feel a little hypocritical if we didn't invite them as Fiance and I started dating when we were 16.)