There are many different combinations of age groups, divisions, and levels. So many sometimes that they can be quite confusing! These were all created to insure that like ages were competing with like skill sets. Levels were designed to make sure that each team competes and is judged by how well they demonstrate a standard set of skills expected for every team competing at that same level.
You do not have to be a certain age to be a certain level. For example, an 8 year old who can do a back tuck is considered a Level 3 tumbler. A 15 year old who can only do a back walkover would be considered a Level 1 tumbler. Teams will be formed by grouping kids in the same age ranges and skill levels. This allows the girls to compete against other teams who are in the exact same boat they are in. The difference becomes how well they execute their skills.
You can visit www.usasf.net for a more detailed breakdown.