I'm down with the general idea of inviting the SO of anyone who is in a relationship.  However, is there some sort of minimum age for the rule?  I'm pretty sure I wouldn't have to invite a high school kid's BF/GF....especially since there's a decent chance they'd split up between STDs/invites and the event.  What about college-aged (in college or not....does that matter?) nieces/nephews?  Do they get +1 for their SO?  They do know other folks since they are all family (in my case).  
When does the rule "kick-in"???