For a rabbit in general, junior prime is in the 4-6 month bracket. For rex, junior prime isn't that great. They come into their adult coat, which is better for show around 6 months old. So show prime for a rex or mini rex is 6 months -1 year, roughly.
Other breeds, depending on the line, some hold their prime longer than others. Like I said, junior prime, which is generally, the shiniest point in the life of the coat, and when the muscles are the tightest, is 4-6 months, and in some lines, pushing up to 8 months. The longer an animal holds prime coat, the more desireable it is.
Then you have senior prime. Some rabbits, once they molt out of junior coat, they are done. I cull those animals. Usually, senior prime, the coat isnt as lusterous, but still nice and shiny. In the case of my tans, most 'settle' in their body mass when they get in the 1-1.5 years old range. Thats when I like to show them the most, because my lines develop slow. Before that, although their coats are better, a lot of times they are too lanky.
I also like a doe that will come back into senior prime after she has a litter. For my tans, I keep does that wean off a litter and within 2 weeks are dried up and have a smooth, shiny senior coat, and no saggy belly muscles. For a rex or mini rex, once a doe has a litter, it is very unlikely she will ever have a coat good enough to show in again. If she has been in a colony, never.
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