I asked the same question recently, about my 30 week old pullet that had stopped laying and was molting. I was told that they go through a mini-molt around 9 months old or so. Their BIG MOLT is at 18 months.
I knew to expect the 1+ year molt, but I had NO idea about the mini-molt for pullets! Perhaps yours are going through a mini, too? You can help get them through the molt faster by upping their protein rations, because those new feathers require a lot of protein.
Another pullet of mine started a mini-molt. But after a couple weeks, I think both birds are nearly done shedding their feathers.
I have not added heat to the coop, btw. They seem to be fine, especially with some extra protein to nourish them.