I think the confusion comes in when you're trying to determine the rabbit's Showability - meaning how well it will compete.
Jr's tend to be placed lower in the class due to lack of maturity in coat, and sometimes even in type (out of balance due to growth). However, if you place a Young Sr (between 6-12months), it will likely place better being in a mature coat and finished any odd ball growth spurts.
There are some Jr's though that do exceptionally well - those are the lucky ones that mature quickly and get a nice finished coat on them by 3-4 months of age. However those also tend to have a quick, nonlasting show career as they don't seem to keep the same showability once they reach Young Sr age. :?
But you can show a Mini Rex at any age as long as it is a minimum 2 pounds, but no more than 3 3/4 poundsas a Jr. Once your bun reaches 6 months, it must show as a Sr. up until it reaches top allowable Sr. weight (that is as long as it makes minimum Sr. Weight).
~Sunshine