Kelly, hi, sure why not? Well, you certainly can't be a freshman or a sophomore anymore.

But now that you're older (and wiser) and in your jr. year, you should be more physically prepared to try out cheerleading. And so what if it's called "varsity" during the junior year? The important thing is you got the spirit and enthusiasm for cheerleading, which is what you like.
I did the same thing during my junior year to try out cross-country running, which I had never try out before. I knew I was up against more experience runners, like in your case, but that just made me want to really focus and work harder at what I want to do and where I want to be.
So, your situation should be similar: your enthusiasm and desire should more than make up for your not having much cheerleading experience.
As long as you went through the try-outs and you made it into the cheerleading team, you are in the door. Now, it's just a matter of working out to show the rest of the cheerleading teammates and your coach you've got what it takes to be just as good and even better than other cheerleaders. Happy POM-POM Cheerleading!

;D