If I were organizing collegiate events at a dance comp, I'd restrict it to people who are dancing with a "Student" USA Dance membership. And the rule I referred to above was related to who can actually get a "Student" membership. Of course, in the absence of a rule saying who is eligible for a "Student" membership, my plan breaks down....
I know that some organizers restrict college events to people who are on a college team, and let the question of eligibility be up to the teams rather than some age/schooling rule such as the one I'm looking for but can't find any more.
The collegiate dancers tend to dislike any kind of "National" rules, preferring to let each college team or association of teams to run things their own way, so although you might prefer to see a consistent rule don't count on it ever coming to pass.