There are different things that can cause a molt, many related to stress. But the molts caused by stress alone are usually not full blown molts but are usually what we call mini-molts. The full blown replace-every-feather-they-have molts are generally caused by the days getting shorter. It is not because they are 18 months old, it’s because the days are getting shorter.
It’s not unusual for a pullet that starts to lay in the summer or fall to skip the molt her first fall/winter and continue to lay straight through to the following fall molt. That’s probably where the 18 month thing comes from. The molt is not caused by them being 18 months old. It’s caused by the days getting shorter and many just happen to be 18 months old when that happens.
A chicken’s feathers get worn out and broken. Winter is a hard time for them to raise chicks. It’s logical that they have set up a system to replace the feathers and stop laying eggs when the days get shorter.
When a hen molts, she quits laying eggs and uses the protein that was going into egg production to make her new feathers. In a full blown molt this can take anywhere from 2 months to 5 months. Some hens are what are called fast molters. They lose a lot of feathers and look practically bald, but this means they are fast molters and will grow their feathers back pretty quickly. Some may still wait until the warmer days of spring to start laying again but many will start to lay whenever the molt is finished. Some hens molt much more gradually. The best sign they are molting is that you see a lot of feathers laying around. It’s really not that easy with some of them to tell if they are molting just by looking at them. These can take a long time to finish.
You can extend their laying hours by providing additional light so they don’t know that the days are getting shorter to trigger a molt. There is a magic number for this that will work for a lot of people but not everyone, 14 hours. You’ll see 14 hours given for this all over this forum. But the key to stop a molt is to keep their days from getting shorter. If you live far enough from the equator, your chickens may have noticed the days getting shorter and started to molt before the days get down to 14 hours long. Chickens close enough to the equator never see days 14 hours long yet they go through the same cycle of molting when the days get shorter.
Just like a chickens feathers get worn and broken, a hen’s internal egg laying factory gets a bit worn out over time. The general egg quality can drop. The shells of brown egg layers can get pretty pale. The whites might get watery. Hatchability decreases. They may lay less frequently. They need some time off to recharge their batteries. Besides, the eggs generally get bigger after they go through a molt. If this did not happen, commercial egg producers would just keep their hens laying for years by controlling the lights. They don’t. When the egg quality and number of eggs drops enough they either put them through a molt or replace them.
Unless you keep the days from getting shorter by adding light, your hens will almost certainly molt and quit laying when the days get shorter if they laid through their first winter. After the molt is finished you can often start them laying earlier by extending the hours of light. You can go with that magic 14 hours if you wish, but just using a timer to turn the lights on an hour earlier than sunrise will usually get them going. And you will really like the eggs you get.
A common strategy is to get a few pullets every year to lay through the winter so you get a few eggs while your older hens are molting. As I said, many pullets will lay through their first winter. That’s generally what I do and it practically always works. But we are dealing with living animals. They don’t come with a guarantee. This past year I had three pullets that did not start laying around 5 to 6 months as I expected but waited until they were 9 months old and started laying in December when the days were shortest and still getting shorter to start laying. They did not read the book that tells them what they are supposed to do or maybe they slept through that chapter. Eggs got really scarce when the older ones went through the molt.
