Your junior status in and of itself should not make your chances worse; however, the state of mind where you have a concept like being "pledge class mommy" will. Age is nothing but a number. Don't dwell on it.
COB events don't mean you personally have a better or worse chance of getting a bid. It is simply a way for all the sororities to get on an even footing numbers wise before formal rush begins. I would go to these events, but DO NOT limit yourself to one or two sororities. Visit ALL the groups that are holding COB events.
ree-xi - first semester freshmen can't rush. The sororities who need to get up to total have an organized sort of COB-fest right before formal rush. This is only for sophomores and up.
http://www.bloomu.edu/greek_life/recruitment
ETA: I just reread the title of this. If you go to Bloomsburg (which we assumed you do from your other post) why on earth are you waiting until fall?