Let me take this opportunity to encourage folks to learn as much as they can about this sport. To that end, there are specific rules about age determination and age divisions for meets in the USAG rules and policies (just published for the 2014-15 season and available to everyone on the USAG web site).
Now to the question -
The competition age is not changeable; it is the age the athlete will be on the last day of the culminating meet for her level for the season (per USAG rules). For level 9's and 10's, the state meet is used for the regular season competition age.
Now, age divisions (age divisions equate to award groups) are a different matter. For an invitational meet (which are most of the meets clubs attend) the age divisions are up to the meet director. An age division can be a literal age (12 year olds) using the competition age above, it can be an "invented" title, like Junior A, which would most likely have a range of birthdays included, or it can be "open" meaning all ages.
I usually try to plan a meet so that my award groups are about equal in size. If that means combining or splitting age groups, then so be it. Lately, I have just started using the method used by USAG for Nationals; I divide the competitors into equal groups and then find the birthday cut-off dates and then title that group whatever I want it to be. On my meet schedule I specify the birthday ranges for the sessions.
Hope this helps.