Most commonly birds molt yearly once they are about 18 months old. There is a huge range of normal in molting and it can vary greatly. Some birds will do an annual molt younger than that. Since they aren't machines, mother nature and their bodies dictate when and how. Usually molt is triggered by the shortening daylight hours, but illness and stress can also be triggers. I've got one that for the last two years has started molt in mid July, and I have some that are only a few weeks into molt now. Your other birds may have started and it could be a very soft molt, which can be almost unnoticeable. Or they may start later. Some birds will do such a soft molt, or even a partial molt that makes it appear they have missed molt completely some years. Sometimes the first clue is a drop in egg production, since they can't make eggs and feathers at the same time, the biggest clue is the feathers everywhere in the coop and run. Upping the protein in the diet can help them get through it, growing feathers takes a lot of protein.
https://www.wideopenpets.com/everything-you-need-to-know-about-molting-chickens/
https://www.wideopenpets.com/everything-you-need-to-know-about-molting-chickens/