Chicks go through two full juvenile molts before they reach final adult size. They outgrow their feathers and need to replace them. Exactly when they go through a juvenile molt seems to vary by chicken, depending on how fast they grow and mature. Molting is not an instantaneous process either, it can go in for a while. I think that is what you are seeing.
I don’t have any great answers for you as to exactly what age they are not molting. I generally process mine sometime after 18 weeks, usually a lot after 18 weeks, but I skin mine and avoids the pin feather problems.