Justhey76-
I would say go for it... I am one of the rare exceptions who pledged my sorority my SENIOR year at age 22!
I attended Emerson College, a college in Boston, MA and joined the local sorority, Kappa Gamma Chi.
In my pledge class of 9, there were 3 seniors and we TOTALLY LOVED IT! I finally had found the sorority that was right for me (I pledged sorority XYZ my junior year, but dropped because the sporority was not what I thought it was).
So if I did at 22 and my senior year AND was accepted with open arms, then I would say a freshman at 20 should go for it! It is never to late to be a sister... trust a late Greek bloomer like myself!
GOOD LUCK