I will go against the prevailing wisdom here and say "go for it" - you say you did it at a university - were you taking a class concurrently? Even if not, if it was truly significant research, I would list it...
But if you have had no research since then, it may beg the question of "why hasn't this applicant done more research since high school?" so it could cast some doubts on your interests in research, assuming that is why you are listing it in the first place...