I think they must meet the minimum weight for a junior. I am not sure, but I think most breeds do have minimum weights. Otherwise, you probably don't want to take anything under 8 weeks to a show as it will be stressful on them and that can cause issues for some rabbits. Many shows will say that all rabbits must meet the minimum weight for the breed and to leave babies at home.
Age is not really a huge factor. It is how the rabbit compared to the standard. You can have younger juniors that can beat seniors. While an 8 week old probably won't win, it could still have better structure than an older rabbit and just hasn't filled out quite enough.
I would weight your rabbit and see if they are big enough. If they are, I would still wait until they are at least 8 weeks old and are comfortable away from the mother and doing well after weaning.
I am no expert though and someone should come along with better info for you.