Yes let’s clear something up. What is the major cause of a full molt? There are a lot of different things that can cause molts, full molts or partial molts. Some of them involve stress of some sort, going without water for a period of time, moving them to a news location, maybe even a predator scare. I’ve had broody hens molt in late summer when raising their chicks. But the main cause of a full molt is the days getting shorter, in the fall in the northern hemisphere. Technically it’s the nights getting longer that triggers it, not the shorter days. These are the chickens not kept under artificial lights. If you provide extra lighting then of course the effect can be different.
Commercial chickens normally go through a certain cycle. By manipulating breeding, lights, and feed they are brought into lay around 5 months of age. Their laying them follows a pattern, production fairly quickly ramps up then slowly declines. At around 13 months of continuous egg production the rate of lay and the quality of the eggs drops off to the point that it’s no longer profitable to feed them and the operator has to decide whether to replace them or force a molt to rejuvenate their bodies. This can vary a little bit but, yes, in general commercial egg laying chickens will molt after about 18 months.
Most of the studies about chickens are performed on commercial chickens, the commercial producers are the ones that can afford to pay for the studies. We can learn a lot from these studies but you can’t blindly read them and assume they apply to a backyard flock. I see that 18 months all over the internet too. Often someone reads one of these studies or hears about it and repeats it, but often loses context. Does this really apply to our backyard flocks?.
If our chicks are hatched in the spring, they usually molt about 18 months later, when the nights get longer. I hatch chicks year around, not just in the spring. I try to observe my chickens. Mine are production breeds, like the OP’s Marans. Decorative breeds can be different. Individual chickens can be different.
As I usually do, I hatched chicks last February. The pullets I kept are laying now and I expect them to continue laying until they molt next fall, probably around 19 to 20 months of age. But I did not keep all of them. One that I did not keep molted this fall, that’s why I didn’t keep her, she did not fit the profile I want in my chickens. Usually I don’t have any of the February hatch molt the first fall/winter, but it occasionally happens. Each chicken is an individual.
I also hatched some chicks this past August. Those pullets haven’t started laying yet though it should not be long. If they follow previous years’ patterns most of those will molt next fall after a spring/summer of laying at about 13 to 14 months of age. If you don’t manipulate the lights, chicks not hatched in the spring typically don’t follow any 18 month rule. They follow the days and nights getting shorter and longer.
Another thing I’ve learned is that you cannot take trends and apply that to one individual chicken. For example, that’s a Buff Orpington, she will go broody. Buff Orpingtons do have a reputation for going broody but a lot of individual Buff Orps never do. Or that’s a sex link, she will start laying at 20 weeks. Some do, some start earlier, some start later. Each chicken is an individual.
Some breeds and just chickens in general do have trends. But you have to have enough for the averages to mean something. One individual Buff Orp may or may not go broody. The more Buff Orps you have the better your odds of at least one of them going broody. One individual chicken is not enough for averages to mean much.
I don’t know how the OP (Original Poster) manages the chickens. I don’t know if the lights are manipulated or not, how they are fed, whether they are laying, especially the Marans in question. I don’t know when that Marans was hatched, just that it is less than 12 months old. There is a lot I don’t know which makes it harder for me to make hard and fast statements like all pullets molt at 18 months, especially when from my experience I know that not all pullets molt at 18 months.
From your other posts I think you are fairly young and have an uncle that keeps chickens. I don’t know of your specific experience with chickens on a day to day basis, how long you’ve kept them, or how you keep them. If you get upset reading this then you get upset. That’s not my goal. I’m trying to get you to think. Don’t blindly believe what you read on the internet, think about how that stuff applies to specific situations. Observe your chickens, but don’t even always believe your eyes. There can be a lot of differences in behaviors between someone that free ranges a flock of hens and roosters compared to someone that keeps a handful of only hens in a small backyard coop and run. Knowledge isn’t especially hard to obtain, it’s how you apply that knowledge that makes the difference.
