If you are a freshman in college, then you have 4 years to contribute to your sorority as a collegian (not to mention all the years as an alumna). I would rush NOW, as a freshman. On some campuses, if you rush as a sophomore, junior or senior, it can become increasingly difficult to get a bid, only because you will be leaving college that much sooner and your absence means one more spot they need to fill that much sooner.
As for you being 20...I don't think anyone will know your age unless you actually tell them your age. Just tell people you're a freshman (which you are) and leave it at that.....
So...I hope you do rush and then come back and tell us all how it went!
Here is my take on it.....
Hybrid Mode
