I rushed as a junior and got a bid. It was definitely a difficult time. As an upperclassman rushing you get cut from a lot of houses based soley on your age. Don't take it personally, because the houses that do invite you back are interested in you for the right reasons.
Everyone gets a Big. She may be older, your age, or younger, but everyone gets a big. My Big was also a junior but was older than me by a few months. I don't think the age of your big matters so much because her main goal is to welcome you into the sorority and answer questions you have about that. Since she's been there in the sorority for a while, it'll be a good fit.
Make sure you be yourself and have fun. My greek friends described the process as being like the sorting hat in Harry Potter. You'll definitely end up where you belong if you trust it