Technically you would go by the weight requirements..however a young jr. usually can't compete with a more developed jr.. So I would go by age and development as well. We usually breed according to the show schedules in order to have jrs./young seniors at a certain age/developement by the showdates.Ideally an older jr of closer to 5 months will compete better. I defiantly wouldn't show under 12 weeks.
3 months of age is where I normally begin, but alot of it depends upon the rabbit's development. Some are late bloomers, and showing them before they're ready doesn't really accomplish much in the grand scheme of things.