By and large. Major League Hacking (the link above) is sort of like the national group "in charge" of hackathons and one of the requirements to be recognized by them is to be a purely under-grad event (though sometimes highschool students are admitted). That said, there are plenty hackathons that aren't sanctioned by MLH and therefore are more open.

It's also possible that my view of hackathons is skewed by the fact that I'm a university student at a school with a very active hackathon culture.