Are they all the same breed? If so, are they all bred from the same lines?
They all start laying when they're ready, and that time will vary a lot depending on many factors. Breed, age, diet, exercise, genetic factors you might not know about because they won't have identical genes even if they are all from the same lines, (like human siblings with the same parents can have completely different traits) weather, climate, how much sunlight they get, how much artificial light they get....
Some won't lay until around 6 months old, or later. I have Brahma hens that didn't lay until nearly a year old. Others, from other lines, may lay much sooner.