pullets has to pass they first year or like you said first winter to be bred from . I did post a article about some diseases related to breeding from young pullets .Here where I live I like for the young Fowl to at least live through one Winter before being used for a breeder.
most of backyard chooks world wide are bred from pullets .a reason all the breed we see are becoming smaller and lighter in weight .place a lot broodiness too .
good breeders will only breed from they older hens and proven breeders .
still waiting to see photos of your old BCM hens one day when they finish molting .