I've heard that overall, chickens dont start molting until 18 months old and in the fall.
I know you read a lot of this stuff on this forum but not really. A major cause of a molt is the days getting shorter, in the fall in the northern hemisphere. The instincts they developed when they were feral and not domesticated by humans was to lay eggs and hatch chicks in the good weather months when food was plentiful. Their feathers wear out and need to be replaced and late fall/winter is not a good time to raise chicks so they stop laying eggs and use the nutrients they were using for eggs to regrow their feathers. The signal for that was the days getting shorter. Then in the spring when food became more plentiful they started laying eggs and raising chicks. The signal for that was the days getting longer.
We've domesticated them and messed with that natural cycle some. We feed them pretty well all year round. We provide good shelter and maybe even extra lights or even heat. We have bred them to be much better egg layers and to go broody a lot less often. In spite of all this they still pretty much maintain their basic instincts but how we manage them can prevent those instincts from kicking in or altering them.
The general trigger for an adult molt is the days getting shorter. Some chicks may be hatched in February, some in September. Not all chickens are 18 months old when the days get shorter and trigger a molt.
Things other than the days getting shorter can trigger a molt. Usually it is stress of some kind. Things like running out of water for an extended period, changes in the pecking order often caused by adding or removing a chicken, moving them into new living quarters or making changes to where they are, a scare like a predator attack or perceived predator attack, a change in lighting. I had a broody hen molt while raising her chicks in mid-summer. She finished the molt about when the other hens started theirs. These can be partial or mini-molts or a full fledged replace-every-feather molt.
Some chickens are fast molters and some are slow molters. That's about how fast the feathers fall out, not how fast they grow back. A fast molter can lose so many feathers they look really ragged with bare spots. A slow molter may lose feathers so gradually you never really notice looking at them but you see extra feathers floating around the coop and run. Then one day their rough worn looking feathers all look new.
I butcher my chickens. It's pretty easy to tell when one is in a molt from the pin feathers and new feathers growing in. Chickens go through what we call juvenile molts. They outgrow their feathers and need to replace them. A fully grown chicken would look pretty silly if all they had were the feathers they had when they were 5 weeks old. From what I've seen with my flock, juvenile molts are generally over with at 4 to 5 months. I'm sure there are exceptions to this, there are always exceptions when people use hard specific numbers when talking about chickens, but I'd be really surprised of they had a juvenile molt at 10 months. They should have stopped outgrowing their feathers long before 10 months.
Some pullets skip their first adult molt the first fall/winter and continue laying all the way until the molt the next fall. Some don't skip that first fall molt, may have a full or partial molt. Some pullets and hens start laying after they have finished the molt without waiting for the warm days of spring or days getting longer. Some wait until the days get longer to start again.
I've gone through all this to say that each chicken is an individual and will react differently than other chickens, even if they are kept in the same conditions. We don't all keep chickens in the same conditions. Each of us is unique in location, climate, flock make-up, how we feed them, management techniques, the list goes on and on. Within our flocks the differences in individuals will give us different results let alone with all our other differences between flocks.
should this be something I should be worried about
In my opinion. not at all as long as they are acting normally. Sometimes chickens in a molt can act a little different. Their feathers growing in can be sensitive to the touch. If you are used to cuddling them they might not like that because their new feathers don't feel good when handled. They may be a little short-tempered with other chickens. If a rooster is molting he may not enjoy mating a hen as much as he normally does, a molting hen might not enjoy that either.
I have not read anything from you that leads me to think you have anything to worry about. When you see changes like that you need to stop and look, it might be something wrong. But I don't see any red flags, it sounds normal. At the same time, since it is a change, have you checked them for mites or lice? Those can sometimes cause chickens to look ragged. Check for roost mites after dark, they hide during the day and attack at night. I don't think this is your problem but it is a reasonable check to make.
