I've got 3 Golden Comets (red sex-links) and an RIR that are 25+ weeks old. None have started laying. The rooster is mating with them but I've heard that doesn't matter.
These 4 are the survivors of 13 we got as chicks. They either had or got something that killed them off one by one in the brooder despite all our efforts (and isolation and cleaning).
Is it possible that whatever disease killed their sisters has delayed or prevented them from laying?