Odd, sounds like your girls are doing at least a partial molt for some reason, even though they are young. Has anything besides the cold weather (which can cause it) happened lately that would be a stress to them, especially if they are all doing it? They are eating and drinking fine in their pen? How much space to they have, lighting? The link below has a list of things that can throw birds into a stress molt.
http://www.thepoultrysite.com/articles/217/moulting-a-natural-process
Stress factors and moulting
Natural moults can occur any time of the year due to birds being subjected to stress. A bird is stressed when the environment or management present a challenge to which the bird cannot respond without suffering a harmful effect. A hen subjected to a mild stress condition in late spring when in full production will suffer a drop in egg production whereas the same stress condition applied to a bird in the autumn will cause her to cease laying and moult.
The following are common stress factors which can induce moulting:
Lighting
decreasing daylight
decreasing artificial light
Loss of bodyweight
Disease
Internal parasites
Climate
excessive cold
heat waves
Feed, feeding and feedstuffs
deficiencies of essential ingredients
irregular feeding
insufficient feed
Predators eg. cats and dogs
Fright - wild birds and children
Peck order - low vitality
Prolonged broodiness
Mismanagement: overcrowding, movement to another house, water deprivation, insufficient feed and water space, faulty ventilation, wet litter, debeaking, vaccinations, exposed housing, etc.
