These were born on January 30th, so they are a little over 3 weeks. I am no expert when it comes to anything rabbit related, but I believe you can see a difference at about 2 weeks and maybe earlier. I have some other litters that are about 6 weeks, and some seem a bit too fluffy but they don't look "messy" like the Astrex- they might just have long coats.
I have not seen it myself, but some Astrex go bald at around 2 weeks, and then regrow their fur. There is also the "eclipse coat" at a later age when they appear to have normal fur before once again having a curly coat.
There isn't a lot of data available on Astrex coats that I have been able to find. Breeders just cull them because their coat is "wrong" according to the standards for all of the recognized breeds. I almost culled Go-Fer-It, my first Astrex doe because of that very reason.
My main goal in raising rabbits is meat, but I also want to breed to the SOP, so when I saw this weird coat pop up my first impulse was to cull her. Now I have decided to pursue a Rex line following the SOP and cull the curly kits to my Astrex program, while maintaining the SOP for Rex, but with an Astrex coat.