You are correct. The point of a big/little program is to pair the new member with an active sister who can be her friend, confidante, and guide in the process of becoming a sister. This doesn't necessarily have anything to do with age or class level; you could end up with a big who is younger than you, but joined the sorority before you did.
Each chapter (or sorority as a national organization) has their own rules and methods for deciding when, how, and to whom they assign little sisters. If you join a sorority you will find out about it once you are an active, until then don't worry about it