I think you can, especially if it's through the university and not something that is typically done by high-schoolers (ie, an undergrad-level activity). You'll list the date, so adcoms can give it the weight it deserves.
1) If you have only been through one or two years of college, list activities from the past 4 years.
2) After that, only list things you have done since the beginning of college.
The summer before high school and college is a bit of a gray zone, but I don't think it would hurt to include the activity on a resume or AMCAS application. Worst case scenario, someone will decide to ignore it.
Thanks for your help. I'm just putting in a lot of work for this (40 hrs/week). Although it's an amazing learning experience, I didn't want it to be a nonfactor when it comes to med school admissions (especially because I plan on applying to the med school that this research is affiliated with).
I know especially if it is continuous from HS on into college, you can count on putting it down. My research started in the middle of HS, and I for sure was not going to just lop off my first two years of research on my AMCAS.
I think you have some lee-way on what to put. The biggest thing is just not to be ridiculous. If it was meaningful and/or very important, you might consider it. Just don't go digging by putting things like National Merit, etc. on your app.