I disagree... all are individuals. Even a leghorn I got and was told would lay by 16 weeks didn't start until 24 weeks.
I never feed layer... dual purpose birds will do best with a little more protein than 16% which is the minimum to sustain light bodies layers like leghorn. GLW is a dp bird... This way my birds who aren't laying as actively or at all can access the OS on the side instead of excess calcium they aren't using, which could delay the onset of lay, stunt growth, or in extreme cases *could* (doesn't mean will) cause gout or kidney failure, if fed long term to birds not in lay.
You can tell if they are maturing or getting close by comb color and behavior. They become more talkative and some will squat when you approach. They also show more interest in the lay box or may begin to practice the egg song. Rest assured, if they aren't hiding a nest.. when they do lay the eggs will be slightly larger size than if they laid earlier. Also chickens are born with all the ovum they will ever have. Laying later maybe means laying longer in life (barring reproductive complications).
Be patient, sometimes it is relative to the time of year they are raised. Other times it's simply slower maturing genetics. Some lines may lay earlier but even among that line their will be some who won't comply with expectation.
