I believe it is by start of season (Nov/Dec) for some meets or before states (late March/April). Either way the kid should be pretty close to 5 by winter time.
According to the usag rules found here: Link Removed
Age Determination and Regulations:
All gymnasts must reach the minimum age for the division before competing in any USA Gymnastics sanctioned competition
So the bronze gymnast needs to be 5 to compete in their first sanctioned bronze meet.