Sexual dimorphism in feathers can be either genetic or hormonal. Many many years ago there were experiments to determine which was which by transplanting areas of skin between males and females -- if the graft later sprouted feathers of a different kind, it was deduced that the sexual differences were due to hormones. If the graft remained the same, it was deduced that it was genetic. And "genetic or hormonal" could relate to different areas of the body -- hackle feathers might be driven by genes, while tail feathers might be driven by hormones.
In any case, when the hormone balance shifts, feathers which are influenced by them will change as well. Capons (castrated male chickens) display plumage differences, as to poulards (spayed female chickens). When females age, they may develop something akin to a condition found in humans called Polycystic Ovarian Syndrome (or PCOS). The ovaries develop cysts, which interfere with ovulatory cycles. Much of the estrogen females have flowing in their bodies arise in developing follicles during ovulation.
There's a bit of a complicated feedback system, whereby the estrogen level regulates the production of a hormone which initiates the chain of reactions that lead to estrogen -- as well as androgens (male hormones). When the mechanism for estrogen production is diminished (as happens in PCOS), the chain of reactions continues longer, which causes androgens to be produced in greater quantities. The result is a shift in the estrogen to androgen ratio, and this can affect plumage, causing male-appearing feathers to start coming in. Usually, by the time this happens, the hen is no longer fertile.
Or, the simpler explanation is that it's comparable to when post-menopausal women not on hormone replacement therapy start getting whiskers.
P.S. The only reason I remember this off the top of my head is because Wednesday I had an exam in my Psychobiology of Reproduction class, and this was one of the topics I had to study.
