Well, it should be much less likely to happen at 1.5 years old. Chickens generally have a better appetite during the colder months, and drink less. Look at the diet of your birds. A good balanced layer ration in addition to a quality vitamin-mineral soluble powder supplement like Avian Super Pack serves as a boost for unbalanced feed rations, or stress from weather, new locations, etc. You've had birds for a year, so I don't mean to inform you as if you are an amateur.
If you see symptoms of respiratory problems, it is best to treat as soon as possible. Tylan is the antibiotic of choice for respiratory problems. Remove birds showing symptoms from the ones that are not and treat them in a separate area that provides them with shelter from weather, food, and water. Tylan 50 is given at 1/2 cc under the skin on the back of the neck, usually with a 25 gauge needle. If the bird is over 5lbs, an additional 1/2 cc should be given in the breast muscle. Birds that are dehydrated are more prone to soreness or reddening of skin at the point of injection. A little irritation may be noticed at the injection site. Do it each day for 5 days. A few drops of Vet-Rx can be added to the water to help the sinuses open.
