There really isn't a set minimum age, but most people won't show rabbits under around 12 weeks. Most breeds to have a minimum weight and really young rabbits usually don't make the weight.
There are 3 possible age categories depending on the breed. Any rabbit under 6 months is a junior, rabbits over 6 months of most breeds are senior, rabbits 6-8 months of certain breeds (large and giant breeds) and intermediate. Mini lops would just have Junior and Senior.
There is no upper age limit, but most rabbits can start to lose some condition as they get older so are not competitive. Some breeds can be more competitive as juniors, so a 1 year old could be 'old' for showing purposes.
If you ever wanted to register her, she needs to be at least 6 months old. There is other criteria though with regards to the pedigree, her weight and not having and DQ's.